Добавил:
Sekretar
kiopkiopkiop18@yandex.ru
t.me/Prokururor I Вовсе не секретарь, но почту проверяю
Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз:
Предмет:
Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_2946_Библиотеки_им_академика_М_И_Перельмана
.pdf
Probability: quantifying uncertainty 25
To avoid the mistake of ignoring prior probability, the clinician must ask, “How common is the hypothesized disease
in my clinical setting?” The commonest error is to overestimate the prevalence of the disease.
Errors inusing therepresentativeness heuristic: using clinical cues that do not accurately predict disease
The cues that make up the textbook description of a disease are imperfect indicators of who has the disease. Cues are
sometimes absent in persons with the disease and sometimes present in persons who do not have the disease. One
mark of an excellent diagnostician is to know how well clinical features predict disease.
Knowing how well the classic features of commonly encountered diseases predict the disease is essential to a lean,
safe style of medical practice. In the next two chapters, we will lay the foundation for understanding the likelihood ratio,
which is the best single measure of predictivity. Past issues of the Journal of the American Medical Association are the best
source of the likelihood ratios of common clinical findings. Since 1998, the journal has published many articles that
Example 1
A long- time resident of Kansas presents to their community clinician with a history of intermittent shaking chills,
sweats, and fever for one week. The physical examination is unrevealing. The examining clinician has just entered
the private practice after having spent two years on the staff of a hospital in Southeast Asia, where malaria is very
common. The clinician estimates that the probability of malaria in this patient is 0.90.
Comment: The clinician has ignored the rarity of malaria in the usual North American patient and has made a
diagnosis strictly on the similarity between this patient and a typical patient with malaria. Based on their preva-
lence in North American patients, other diseases are far more likely than malaria to be the cause of fever, shaking
chills, and sweats.
Example 2
A 35- year- old woman with mild hypertension is obese and has prominent striae and moderately excessive facial
hair. She does not take corticosteroids. The medical student clerk has just completed an endocrinology elective.
They immediately suspect Cushing’s disease and tell their preceptor that there is a 30% chance that the patient
has this disease. They had written an order for a complete battery of tests of adrenal function. Their preceptor
cancels the order.
Comment: Cushing’s disease is the cause of hypertension in fewer than one in 100 patients. Features of Cushing’s
disease occur in other conditions which are much more prevalent. Even when the patient’s clinical features are
quite representative of classic Cushing’s, the diagnosis is still a long shot.
Example
A previously healthy patient comes to the emergency department with the sudden onset of shortness of breath.
The clinician on duty initially suspects a pulmonary embolism but discards the possibility because the patient
shows no signs of blood clots in the leg veins. Furthermore, the patient does not complain of coughing blood or
chest pain. The clinician sends the patient home. Two days later, the patient’s shortness of breath worsens, and
the patient goes to another hospital where the emergency department clinicians correctly diagnose pulmonary
embolism.
Comment: The clinician who first saw the patient has underestimated the probability of pulmonary embolism
because the patient did not have two of the classic features of pulmonary embolism. The clinician did not know
that only one- quarter of patients with pulmonary embolism cough blood and only one- third have clinical evi-
dence of blood clots in the leg veins. These findings, while part of the classic description of pulmonary embolism,
are not reliable clues to the disease.
https://t.me/medicina_free
26 Medical decision making
summarize how well the clinical findings of common diseases predict the disease. The feature is called The Rational
Clinical Examination, which is also the name of the book that summarizes this series of articles (see the Bibliography).
Errors inusing therepresentativeness heuristic: being too sure ofa diagnosis when redundant
predictorsarepresent
When a patient has many of the classic predictors of a disease, the clinician is often very confident of the diagnosis
because “the story holds together pretty well.”
However, internal consistency does not necessarily lead to accurate predictions. Consider this extreme case. If the
classic predictors of disease always occur together, knowing that one predictor is present is the same as knowing that
all are present. For purposes of diagnosis, the other predictors are redundant. They do not add information, and the
clinician should not be any more confident of the diagnosis when several features are present than if only one feature
is present. Clinicians often assume that each additional finding increases the probability of disease proportionately. In
fact, clinical cues may be less predictive in combination than the clinician expects.
Empirical evidence about which disease cues are uncorrelated predictors of disease (i.e., independent) is increas-
ingly available. This information often is presented in the form of models for using clinical findings to predict disease.
We discuss these clinical prediction models later in this chapter.
Errors inusing therepresentativeness heuristic: mistakenly using regression tothe mean
asdiagnostic evidence
A change in the patient’s condition is often used to test a diagnostic hypothesis:
A therapeutic trial consists of giving a treatment that relieves the symptoms of a hypothesized disease but is ineffec-
tive in other diseases. If the patient improves, the clinician’s estimated probability of the disease increases accordingly.
The drug’s mechanism of action leads us to expect that it would affect the pathophysiology of the disease, and so the
change in the patient’s status becomes a diagnostic feature of the hypothesized disease. A response to specific treat-
ment, therefore, increases the match between the patient’s features and the classic features of the disease.
The test of time is another form of therapeutic trial. Here, the clinician withholds treatment to test the hypothesis
that the patient does not have a serious disease. If the patient improves without treatment, the probability of serious
disease goes down. The response to the test of time also improves the match between the patient’s features and the
typical features of a self- limited illness. This type of hypothesis testing is often used in clinical practice.
However, changes in disease status that coincide with a therapeutic trial may mislead the clinician because they are
due to random variation in the course of the disease rather than cause- and- effect. The name of this relationship is
regression to the mean.
Example
A 40- year- old woman has chest pain that is retrosternal, radiates to the left arm and is crushing, squeezing, and
pressure- like. The clinician concludes that the pain is indicative of coronary artery disease and admits the patient
to the cardiac care unit. The patient is discharged the next day with an appointment for a test of her gallbladder
function.
Comment: The patient appeared representative of coronary artery disease, and her pain is indeed anginal in
quality. But the probability that she had coronary artery disease would have increased considerably if she had one
or more of the following independent predictors of coronary artery disease. Each one adds information even
when the others are present.
• A history of pain brought on by exertion and emotional stress
• Pain relieved promptly by rest or nitroglycerin
• Pain so severe that the patient had to stop all activities when the pain occurred
• A history of smoking cigarettes for many years
These findings are independent predictors of coronary artery disease. We know this because a multivariate
regression analysis (a statistical method) of patients with chest pain showed that each of them increased the prob-
ability of coronary artery disease regardless of the presence of the others. In contrast, the location, radiation, and
descriptors of anginal pain tend to occur together and are therefore highly correlated.
https://t.me/medicina_free
Probability: quantifying uncertainty 27
What is the basis for this example? Most biological measurements vary randomly over time in an individual. When
these random values are symmetrically distributed about a mean value, the result is called the normal distribution
curve (Figure3.4).
The shape of the normal distribution curve shows that events whose value is close to the mean are much more com-
mon than events whose value is far from the mean. Therefore, an extreme value is more likely to be followed by a value
that is closer to the mean than a value close to the initial value.
In the upper panel, a healthy person has an unlikely, surprisingly high, test result. A repeat test gives a much more
likely value, given the shape of the distribution curve in healthy people. The lower panel depicts a surprisingly low
value for a diseased person. A repeat test gives a much more likely, higher value. A high value is more likely than not
to be followed by a low value. The clinician may therefore misinterpret a random event such as the fluctuations in
symptoms of a minor illness as proving the success or failure of a therapeutic trial.
Errors inusing therepresentativeness heuristic: comparing apatient toa small, unrepresentative
experiencewitha disease
When clinicians use the representativeness heuristic to judge the probability that a patient has a disease, they often
compare the patient to their personal experience with the disease. In doing so, they often neglect to account for the size
of their personal experience. When personal experience with a disease is limited, the principles of statistical sampling
tell us that it is likely to be atypical. A patient with atypical clinical features may match the clinician’s small, atypical
experience of patients with a disease, leading the clinician to conclude that the patient is highly likely to have the
disease.
Why is a small experience likely to be atypical? A clinician’s personal experience with an event is a sample of the
universe of all such events. From statistical theory, we learn that a small sample is more likely to deviate from the par-
ent population than a large sample. Thus, a small personal experience may be quite unrepresentative of the parent
Example
A patient has mild hyperglycemia on a single measurement of serum glucose. The clinician puts the patient on a
diabetic diet. The patient’s blood sugar falls, and the clinician concludes that the patient had diabetes.
Comment: The patient may not have diabetes. Hyperglycemia on the first blood glucose test could be a random
variation in this biological measure. The second measurement was simply a less extreme sample from the same
frequency distribution of serum glucose for this patient.
No. of
patients
No. of
patients
Numerical result of test
Numerical result of test
Repeat of test
in a healthy person
Repeat of test
in a diseased person
Repeat
Repeat
Initial
Initial
Figure3.4 Two examples of regression to the mean: in normal individuals (top panel) and in diseased individuals (bottom panel). The curve on the
left corresponds to the random distribution of results in a normal individual. The distribution on the right corresponds to results in a diseased
individual.
https://t.me/medicina_free
28 Medical decision making
population. An event which is unusual in the parent class, and therefore improbable, may be judged probable because
it is representative of a clinician’s small personal experience.
Mistakes in using the representative heuristic are avoidable. Clinicians should rely more on published accounts of
the typical features of diseases and less on personal experience. Knowing more about the prevalence of diseases in
one’s practice would help, as would a wider exposure to patients and their resulting diagnoses. The best way to
become an expert diagnostician is to see lots of patients.
This example concludes our discussion of the representativeness heuristic. Time spent on learning about the repre-
sentativeness heuristic will be well repaid. Clinicians use this heuristic many times each day in clinical practice.
Clinicians could improve their clinical reasoning skills by remembering these examples.
The next heuristic is much easier to understand than the representativeness heuristic.
3.3.3 Heuristic II: availability
Availability, the process by which repetition enhances recall, is a second heuristic for using personal experience to
determine probability.
Availability is a valid clue for judging probability, owing to experimental evidence that frequent events are easier to
remember than infrequent events. However, other factors also affect the ease of recall. They include vividness, the
consequences for the clinician or the patient, immediacy, recency, and, paradoxically, rarity. Making a difficult diagno-
sis is deeply satisfying, and its memory persists. For the same reason, clinicians remember the patient with a rare dis-
ease. When examining a patient with an unusual pattern of findings, a clinician may vividly recall a patient with
similar findings and an unusual disease. Clinicians overestimate the frequency of these special events in their profes-
sional life because their memories of them are so vivid. They remember the patient with a rare or difficult- to- diagnose
disease but forget the many patients with similar findings who had more commonplace diseases.
Example
Dr. V’s patient has a heart rate of 100/min, has lost a little weight, and has been irritable of late. Although there is
no enlargement of the thyroid gland, Dr. V estimates that the probability of hyperthyroidism is 0.50 because the
patient closely resembles the only two cases of hyperthyroidism that Dr. V has diagnosed in ten years of primary
care practice. When thyroid tests are all normal, Dr. V can scarcely believe the results and sends the patient to a
consultant to help resolve this unusual case.
Comment: In a large population of hyperthyroid patients, 95% will have an enlarged thyroid gland. Dr. V’s
personal experience was too small to be representative of hyperthyroidism.
Definition
Availability Heuristic judging the probability of an event by how easily similar events come to mind.
Example 1
A clinician overestimates the probability that a patient with diarrhea has amoebiasis because of a recent patient
who had amoebiasis (which is an unusual cause of diarrhea in the United States).
Example 2
A clinician recently made a first- ever diagnosis of a subphrenic abscess by doing a white cell scan on a patient
with fever and abdominal pain. For several months, every patient with abdominal pain and low- grade fever had
a white cell scan to rule out a subphrenic abscess. The patients all recovered uneventfully in a few days and won-
dered why they had to have an expensive test.
https://t.me/medicina_free
Probability: quantifying uncertainty 29
The third heuristic is the mental process by which special characteristics of the patient are used to estimate
probability. This heuristic is called anchoring and adjustment.
3.3.4 Heuristic III: anchoring andadjustment
Clinicians often assess probability starting from an initial probability and arriving at a final probability by adjusting to
take account of the individual features of a patient. For example, they start with a probability based on the prevalence
of the target condition and adjust upward or downward based on the individual patient’s findings. This very impor-
tant heuristic is often used incorrectly. According to experimental evidence, the usual mistake is to adjust too little after
acquiring additional information. The bias toward the initial probability estimate is a failure of adjustment. The anchor
depicted in Figure3.5 represents the cognitive resistance to adjusting the starting probability. This heuristic for assess-
ing probability is biased because people tend to place too much emphasis on the starting point (the first impression)
and too little on new information.
There are several reasons why the anchoring and adjustment bias leads to incorrect adjustment of probability
estimates.
• People tend to overestimate the probability of events that are defined by two or more features occurring at the same
time. This type of event is called conjunctive because the events have an apparent connection with each other.
Theerror in estimating probability may be due to overconfidence in redundant cues, one of the misuses of the
representativeness heuristic.
• People tend to underestimate the probability of an event that is defined by one and only one of several features
occurring. This type of event is called disjunctive because the events lack an apparent connection with each other.
• When asked to describe their uncertainty about a probability, people tend to over- state their certainty, as manifest
by a narrower distribution of probabilities than is consistent with their stated level of certainty.
• People make a subjective judgment about how much to adjust a probability after getting new information. Research
shows that their adjustment is closer to the starting point than it should be as determined by the post-
test probabil-
ity calculated with Bayes’ theorem.
Bayes’ theorem, which is the topic of the next chapter, is an unbiased approach to adjusting probabilities from a start-
ing point. Bayes’ theorem indicates exactly how much to adjust an initial probability when additional information
becomes available.
0
1.0
Probability of disease
Adjustment
Special
attributes
of a patient
Large personal
experience
Published
experience
Figure3.5 Schematic depiction of anchoring and adjustment.
Example
A patient with chest pain has atypical angina, and the clinician estimates the probability of coronary artery dis-
ease to be 0.70. The clinician orders an exercise electrocardiogram, which is very abnormal. Instead of diagnosing
coronary artery disease, the clinician orders a costly cardiac CT angiogram.
Comment: When the probability of disease prior to the test is estimated to be 0.70 and the exercise electrocardio-
gram is very abnormal, the probability of coronary artery disease is at least 0.95. At this point, most clinicians would
tell the patient that they had coronary artery disease and discuss the choice between medical treatment or revas-
cularization. Relying on intuition rather than Bayes’ theorem, this clinician underestimated the effect of a very
abnormal exercise ECG on the probability of coronary artery disease and ordered an unnecessary confirmatory test.
https://t.me/medicina_free
30 Medical decision making
3.3.5 Correctly using heuristics forestimating probability
Using personal experience to estimate probability is among the most important topics in this book because it happens
many times in a day in the practice of medicine. The student of medicine must learn how to avoid the pitfalls of the
methods that people use to estimate the probability, which requires a secure understanding of the heuristics for recall-
ing the experience.
Making precise, unbiased probability assessments from personal experience is beyond the cognitive ability of almost
everyone. Clinicians do their best, and their best is often astonishing. Nonetheless, the wisest among us seek guidance
from published experience, which is our next topic.
3.4 The role ofempirical evidence inquantifying uncertainty
Earlier in this chapter we defined probability as a quantification of judgment about the likelihood of an event. In the
preceding section, we focused on the use of judgment to determine a patient’s probability. We referred to pat-
terns of clinical features that comprise a description of the disease. Other than the iterative process called indi-
rect probability assessment, we said nothing about how to use judgment to quantify one’s uncertainty. The
processes to be described in this section will determine a number that can inform the judgments that establish a
probability.
The italicized phrase may seem less obscure if you remember the anchoring and adjustment heuristic. A probability
assessment often starts with a number that represents a broad population. Clinicians arrive at a final probability by
adjusting the starting probability to take account of the individual features of a specific patient. Published experience
is often the source of that starting number. The mortality rate from an operation, the probability of an adverse effect of
therapy, and the prevalence of a severe form of coronary artery disease are examples of starting probabilities obtained
from published studies. Published studies are useful for several reasons:
A published report usually reflects a much larger experience with a disease than most clinicians see in a lifetime in practice.
A published report is the product of systematic study and unbiased reporting according to the fundamental principles of
epidemiology.
Statistical analyses in published studies often organize the findings in a form that is useful for determining a specific patient’s
probability.
Published studies often report prevalence, which is the proportion of a population that has a characteristic at a specified
instant in time (e.g., the proportion of 23- year- old men in the population of a town on a certain date). The prevalence of
a disease can be a starting point for determining its probability in a specific patient. Do not confuse prevalence with
incidence, which is the number of occurrences during a specified period of time (e.g., the number of men in a defined
population who had a stroke during one year).
Prevalence can also mean the proportion of a subgroup defined by a formal process for combining clinical findings
(i.e., a clinical prediction model), which is our next topic.
In the next several pages, we describe three ways to form a group of patients in which to measure the prevalence of
a disease. In each, the common feature is a diagnostic problem.
3.4.1 Determining probability fromthe prevalence ofdisease inpatients witha symptom,
physicalfinding, or test result
The prevalence of a disease in patients who have a symptom, physical finding, or diagnostic test result helps a clinician
to diagnose the disease (“common diseases are common”).
Example
A medical student evaluates a young man with abdominal pain. Their main concern is appendicitis. The pain is
present throughout the abdomen and is associated with loose bowel movements. The patient does not have local-
ized abdominal tenderness, fever, or an increased blood leukocyte count. The medical student presents the pa-
tient to the chief surgical resident who, to the student’s surprise, discharges the patient from the emergency room.
https://t.me/medicina_free
Probability: quantifying uncertainty 31
A published prevalence of disease may not apply to a specific clinical situation. The most common problem is sys-
tematic differences between the published article’s study population and the specific clinical situation. For example,
the prevalence of renovascular hypertension in a specialty clinic is higher than in a primary care practice. A second
problem is a small sample size, which means imprecise prevalence estimates that would propagate through subse-
quent calculations of post-
test probability.
3.4.2 Determining theprobability ofa disease fromits prevalence inpatients witha clinical syndrome
After evaluating a patient, the clinician may identify a clinically defined syndrome (e.g., nephrotic syndrome) whose
causes, and their prevalence, are known from published studies. This process sounds straightforward, but the preva-
lence of a disease in a syndrome may depend on how the researchers assembled the study population. For example,
the prevalence of the diseases may be higher in hospital patients than in office-
based practice. Therefore, deciding that
the prevalence in a published study applies to an individual requires attention to the match between the study setting
and the clinical setting in which the individual was seen.
The opportunities for using the prevalence of disease in patients with a clinical syndrome are limited because of a
dearth of published information on the prevalence of diseases in clinical syndromes.
Definition
Syndrome: a collection of signs and symptoms that consistently occur together and are associated with one or more
specific diseases.
Example
A resident is examining a 45- year- old man who came to the emergency room after experiencing retrosternal chest
pain for the first time earlier in the day. The pain was pressure- like in quality and was confined to the chest. The
pain came on after a hurried meal and lasted about ten minutes. He has felt fine since then.
The resident is trying to decide whether to test for coronary artery disease. They know that the decision about
testing should depend on how well the patient’s history fits the typical history for exertional angina pectoris,
atypical angina, or non- anginal chest pain. The resident looks up an article which indicates the prevalence of coro-
nary artery disease in each of these syndromes. After consulting with a cardiologist, they decide that the patient’s
history is most consistent with non- anginal chest pain, counsels the patient against eating too fast, and schedules
a follow-
up visit in two weeks.
Comment: This hypothetical study illustrates some of the problems of studies of disease prevalence in patients
with a syndrome like chest pain. The study may lack standardized criteria for assigning a patient to one of the
three chest pain syndromes. A related study design problem: the placement into one of the three syndromes
depends on one clinician’s diagnosis. A better study design would ask two clinicians to independently classify
each study patient’s chest pain syndrome. They may get the same history and differ in its interpretation or obtain
different descriptions of the illness and classify the patient’s history differently. A well-
designed study would
have the two clinicians compare their diagnoses and try to resolve any differences.
Comment: The chief surgical resident knows that the prevalence of appendicitis among self- referred adult males
with abdominal pain is only 1%. The student should use this information as a starting point as they use the
patient’s clinical findings to determine the probability of appendicitis. If they do not suggest appendicitis, the
probability of appendicitis is very low since it was 1% in the average male with abdominal pain. If the examina-
tion does suggest appendicitis, the student’s probability must reflect the low prevalence of appendicitis in all men
with abdominal pain.
https://t.me/medicina_free
32 Medical decision making
3.4.3 Establishing aprobability using aclinical prediction model
Clinical prediction models use clinical features and other information to estimate a patient’s probability of the target
condition. This probability is the starting point for using the anchoring and adjustment heuristic to determine the
patient’s probability of the target condition. Most clinical prediction models are empirical. A typical study uses the
following process.
1. Researchers obtain each item on a list of prespecified clinical findings from many patients with the same clinical
symptom, sign, or test result. The researchers then establish the final diagnosis by a method that is independent of
the clinical findings (e.g., a coronary arteriogram).
2. The researchers identify the predictors of disease by statistical methods that assign to each predictor a weight that
takes the influence of the other predictors into account so that its weighting does not depend on whether the other
predictors are present or absent.
3. The prediction model uses an explicit method for assigning patients to diagnostic subgroups based on their find-
ings. For example, each patient has a score that is the sum of the weights of the patient’s findings; patients with
similar scores are assigned to the same subgroup.
4. The prevalence of the target condition in a diagnostic subgroup is the number of patients in the subgroup with the
target diagnosis divided by the total number of patients in the subgroup.
5. The prediction model must be tested on additional patients to verify the prevalence of disease in the subgroups.
The principal statistical methods for clinical prediction models are regression analysis and recursive partitioning.
Regression analysis
Regression analysis describes the relationship between predictors (the independent variables or predictor variables)
and the predicted event (the dependent variable). Regression analysis shows how the dependent variable changes
when the value of a predictor changes while holding constant the values of the other variables. Regression analysis
tests the hypothesis that a predictor variable is related to the dependent variable. It addresses the following question:
“Independently of the other potential predictors, is the association of this predictor with the dependent variable real
or due only to chance.” Typically, a regression analysis shows that when all other candidate predictors are taken into
account, some predict and some do not.
Regression analysis assigns a numerical weight to each predictor. The weight is a measure of how well the predictor
discriminates between different values of the dependent variable (i.e., does the patient have the target diagnosis, or
not). The larger the weight assigned to a predictor, the better it discriminates and the greater the change in the depend-
ent variable when the predictor is present.
The weights have practical value in diagnosis. To use a prediction model derived by regression analysis, the clinician
determines whether a predictor is present (e.g., asks a patient whether exertion causes the patient’s chest pain). The
clinician adds the numerical weights corresponding to the predictors that are present. The sum of the weights is a
score. We will use the term discriminant score to represent this sum. The discriminant scores for diseased and non-
diseased patients are distributed differently, as shown in Figure3.6 for a hypothetical example.
As seen in Figure3.6, the discriminant scores of diseased patients overlap with the scores of non- diseased patients.
The researchers may choose a cutoff score below which most patients are not diseased and another cutoff score above
which most patients are diseased. Between these two cutoff scores, the prevalence of the disease is intermediate. The
clinician uses a patient’s discriminant score to put the patient into a group, as shown in Table3.1. This partitioning of
patients may be linked to different actions: do nothing, get more information, or start treatment. Chapter13 takes up
this topic in depth.
No. of
patients
Discriminant or logistic score
Disease present
Disease absent
123
Figure3.6 Hypothetical distribution of discriminant scores for diseased and non- diseased patients. The cutoff scores are represented by vertical
lines. “Logistic score” refers to a regression analysis in which the dependent variable is binary (e.g. disease present or absent).
https://t.me/medicina_free
Probability: quantifying uncertainty 33
Alternatively, the researchers may divide the range of scores into equal- size segments (e.g., scores of 0–3, 4–6, 7–9,
and so forth). The probability of disease assigned to a subgroup by a patient’s discriminant score is the prevalence of
disease in that discriminant score subgroup. As a reminder of our terminology, this prevalence is an objective probability;
its basis is a study of a population of patients.
Table3.1 Clinical prediction models scores and their meaning.
Score group Range of scores Probability of disease Action
1 Low Low Do nothing
2 Intermediate Intermediate Do a diagnostic test
3 High High Start treatment
Example 1 Chest pain inreferral practice
One of the authors asked 208 patients who had been admitted to the hospital for elective coronary arteriography
to answer a set of questions about their chest pain and medical history. Seven findings in the history discrimi-
nated between patients with significant narrowing of at least one coronary artery and patients with no significant
narrowing (Table3.2).
Table3.2 Empirical clinical prediction rule forcoronary artery disease.
Attribute Diagnostic weight
Age >60 years +3
pain is brought on by exertion +4
Patient must stop all activities when the pain occurs +3
History of myocardial infarction +4
pain relieved within 3minutes after taking nitroglycerin +2
At least 20 pack- years of cigarette smoking +4
Male gender +5
Adapted from Sox etal. (1990).
Example 2 Chest pain inprimary care
The researchers from five studies of chest pain in primary care practice combined their study patients into one
large data set from which they developed a clinical prediction model. They found seven independent predictors
of a CAD diagnosis; because the weights were similar, they rounded them to +1 and −1 (Table3.3). Note that one
predictor has a negative weight; it reduces the probability of CAD.
Table3.4 shows the number of patients with each chest pain score and the corresponding prevalence of CAD
(see next page). Only 75 of the 644 patients (12%) had a CAD diagnosis.
Table3.3 Chest pain predictors or coronary artery disease inprimary care.
Clinical predictor Weight of finding
Pain reproduced by palpating the chest wall −1
Older age (male
≥ 55 years; female ≥ 65 years) +1
Physician initially suspected a serious condition +1
Chest discomfort feels like “pressure” +1
Chest pain is related to physical effort +1
History of CAD +1
Chest pain score range −1 to +5
Adapted from Aerts etal. (2017).
https://t.me/medicina_free
34 Medical decision making
Recursive partitioning
Recursive partitioning is a statistical process that leads to an algorithm for classifying patients.
The first use of clinical algorithms was to display the logic of diagnosis for medical corpsmen in the military, physi-
cian assistants, and nurse practitioners. Their use to describe a diagnostic or treatment strategy has spread to standard
textbooks of medicine and journal articles. A person can use an algorithm to describe their own logic in solving a prob-
lem. The first clinical algorithms were based on clinical judgment. Algorithms can be based on a study in which
researchers obtain prespecified clinical findings from many patients and then use other means to establish the final
diagnosis in each person.
In analyzing a dataset of such study patients using recursive partitioning, the diagnostic process is represented by a
series of yes- no decision points. A finding that is associated with the final diagnosis places the patient in one group; if
the finding is not present, the patient is placed in a second group. The patients in each of the two groups are asked a
second yes-
no question about another finding. This process continues until it reaches a predefined stopping point. The
goal of the process is to place each patient into a group in which the prevalence of the disease is either very high or very
low. Typically, the finding at each yes- no decision point best discriminates at that point in the partitioning process
between those with the target condition and everyone else.
Evaluating theperformance ofclinical prediction models
Clinical prediction models are based on the systematic study of patients. They reflect the accuracy of clinical findings
in the real world of patient care, which makes them a strong foundation for determining a patient’s probability.
However, clinical prediction models can lead to incorrect probabilities, which could lead to poor decision making and
clinical outcomes. Prediction models must be tested before they are used in clinical practice.
The principal problem is over-
fitting. The statistical techniques optimize discrimination in the patient population
that was used to create the model (the training set). When the model is used in other populations (the test sets), it typi-
cally discriminates less well. This outcome is known as over- fitting. Too many candidate predictor variables and too
few patients in the training set are related causes of over- fitting. A good general rule: the training set should contain at
least 10 patients with the target condition for every candidate predictor variable. A study with 10 candidate predictors
and 50 training set patients with the target condition would fail this rule; one with 200 such patients would pass. A
likely explanation for over- fitting? Small samples of patients are likely to be atypical.
A good general rule: Do not use an untested clinical prediction model!
The best way to test a prediction model is to apply it to patients from a different setting than the training set patients,
establish the final diagnosis, and calculate measures of discrimination and calibration. Discrimination is the ability of
the model to distinguish patients with the target diagnosis from everyone else. The c- statistic gives the probability that
a patient with the target diagnosis will have a higher discriminant score than a patient with other diagnoses. A c- statistic
of 0.50 denotes no discrimination; a c- statistic of 1.0indicates perfect discrimination. Calibration is a set of techniques
Table3.4 Clinical prediction models scores and their meaning.
Chest pain score –1 0 1 2 3 4 5
Patients with score
and CAD/Patients
with score
0/87 1/208 6/160 11/85 29/53 21/32 17/19
Probability of CAD
(95% confidence
interval)
0.00
(0.00–0.03)
0.00
(0.00–0.02)
0.04
(0.01–0.07)
0.13
(0.07–0.21)
0.55
(0.41–0.67)
0.66
(0.49–0.80)
0.89
(0.71–0.98)
Adapted from Sox etal. (2019).
Definition
ALGORITHM: Step- by- step instructions for solving a problem.
https://t.me/medicina_free
Соседние файлы в папке Библиотека им академика М.И. Перельмана