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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_2593_Библиотеки_им_академика_М_И_Перельмана
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Figure 2-1 Bilirubin metabolism.
Plasma
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When bilirubin is reported by the lab, it is usually reported as total
bilirubin, which consists of both conjugated and unconjugated
fractions. If there is an elevation in any of the fractions, it will lead to
an elevation of total bilirubin concentrations. It is important to note
that with the exception of neonates, the majority of bilirubin should
consist of the conjugated fraction.
61
MISCELLANEOUS TESTS
Amylase and Lipase
Amylase (reference range: 35–118 IU/L or 0.58–1.97 μkat/L) and
lipase (reference range: 10–160 IU/L or 0–2.67 μkat/L) are enzymes
produced by the pancreas and secreted into the duodenum to assist
in the digestive process. Small amounts of both enzymes are also
found in the saliva and stomach. Significantly elevated levels of
either enzyme are suggestive of pancreatic damage.
Amylase is responsible for breaking down complex carbohydrates
into simple sugars. Significant elevations in serum amylase are
observed in patients with acute pancreatitis or pancreatic duct
obstruction. Amylase levels tend to rise 6 to 48 hours after onset of
the disease and usually return to normal 3 days after the acute
event. In chronic pancreatitis or obstruction, amylase levels may
remain elevated for longer periods. Other nonpancreatic conditions
(eg, bowel perforation, biliary disease, perforated peptic ulcer,
ectopic pregnancy, and mumps) can be associated with elevated
serum amylase levels.
Lipase is responsible for breaking down triglycerides (TGs) into
fatty acids. Elevated serum lipase levels are also suggestive of
pancreatic disease and tend to be more specific for pancreatic
disease than for amylase. Nonpancreatic conditions such as
gallbladder disease or biliary cirrhosis can also lead to elevated
lipase levels. The onset of lipase elevation is similar to that of
amylase; however, lipase typically remains elevated for 5 to 7 days,
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and can be useful in diagnosing patients in later stages of pancreatic
disease. Narcotics (eg, morphine) can constrict the sphincter of Oddi
and increase serum concentrations of both amylase and lipase.
Prostate-Specific Antigen
Reference Range: 0–4 ng/mL or 0–4 μg/L
Prostate-specific antigen (PSA) is a protease glycoprotein produced
almost exclusively by prostate epithelial cells. Large quantities of
PSA are carried in semen; only low levels are found in the blood.
Serum concentrations of PSA are increased when the normal
prostate glandular structure is disrupted by benign or malignant
tumor or inflammation (prostatitis). More than half of males with
benign prostatic hyperplasia (BPH) have elevated serum PSA
concentrations. PSA is also a valuable parameter for staging and
monitoring the progression and response to therapy of prostate
cancer.
64
The prostate gland increases in size with age; therefore, it is
expected that older males will have higher PSA levels compared with
younger males. PSA serum concentrations can also increase after
prostatic manipulation such as digital rectal examination (DRE),
catheter placement, transrectal ultrasound, cystoscopy, or biopsy of
the prostate. In addition, serum PSA will increase 24 to 48 hours
after ejaculation. Although elevated serum concentrations of PSA
can occur in males with BPH, concentrations tend to be higher and
encountered more often in males with cancer. Males with PSA levels
between 4 and 10 ng/mL should be evaluated further for potential
prostate cancer.
The serum half-life of PSA is 2 to 3 days, but serum PSA
concentrations can remain high for several weeks after manipulation
of the prostate. Circulating serum PSA is bound to plasma proteins,
and the capability exists to measure both total and free (unbound)
PSA concentrations. Increased risk of prostate cancer has been
observed in males with a free PSA to total PSA ratio of <0.25.65 An
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aggressive approach to localize prostate cancer for males with life
expectancies >10 years is now favored.
64,66
Thyroid-Stimulating Hormone
Reference Range: 0.5–4.7 μIU/mL or mIU/L
Thyroid-stimulating hormone (TSH, also known as thyrotropin) is
secreted by the pituitary gland to stimulate the thyroid gland to
produce the thyroid hormones T4 and T3. TSH is measured, often in
conjunction with the thyroid hormones, to diagnose thyroid disorders
and to monitor exogenous thyroid supplementation therapy. The
reader is referred to Chapter 52, Thyroid Disorders, because it
provides a more detailed discussion of the clinical implications of
altered thyroid laboratory findings.
Procalcitonin
Procalcitonin is a precursor of calcitonin and is typically undetectable
in healthy individuals. Elevations in procalcitonin occur in patients
with inflammation secondary to bacterial infections; however, a
similar increase is not observed in patients with inflammation
secondary to viral infections or noninfectious conditions. Once there
is a systemic inflammation due to bacterial infection, synthesis of
procalcitonin will be induced in almost all tissues and then released
into the blood.67 Interestingly, increases in calcitonin are not seen in
patients with elevated procalcitonin. In patients with sepsis or sepsis
syndrome, procalcitonin levels <0.5 ng/mL have been associated
with a low risk of progression to severe sepsis, and levels >2.0
ng/mL represent a high risk for severe sepsis. Trials involving lower
respiratory tract infections have suggested that antibiotic therapy
should be discouraged in patients with procalcitonin levels <0.25
ng/mL but encouraged for those with levels ≥0.5 ng/mL. These
criteria have also been used as a guide for discontinuing therapy as
infections resolve; however, there are differences in how
procalcitonin levels are managed among different institutions.
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Additional further trials will help accurately define the role of
procalcitonin testing.
Cholesterol and Triglycerides
A detailed discussion of hypercholesterolemia and lipid disorders is
provided in Chapter 8, Dyslipidemias, Atherosclerosis, and Coronary
Heart Disease. For convenience, the current range of desired values
for total cholesterol (TC), low-density lipoproteins (LDLs), highdensity lipoproteins (HDLs), and fasting TGs has been incorporated
in Table 2-2.
HEMATOLOGY
There are several different hematologic cell types that originate from
the hematopoietic stem cell. Each cell line has a defined role and
unique contribution to the overall homeostatic process and may be
found in the bone marrow, lymph system, or blood. Typically, routine
clinical laboratory testing involves measuring concentrations of
mature myeloid cells found in the blood. Figure 2-2 illustrates the
various lineages derived from the hematopoietic stem cell.68 The
cells derived from the myeloid linage are the focus of the following
discussion. Readers are encouraged to refer to Section 16,
Hematology and Oncology, to gain further understanding of the
clinical relevance of lymphoid and myeloid cells (Fig. 2-2).
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Figure 2-2 Hematopoietic stem cell lineage. MK, megakaryocyte;
EB, erythroblast. (Adapted with permission from Greer JP, Foerster
Bone
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Lippincott Williams & Wilkins; 2009:80.)
Complete Blood Count
The complete blood count (CBC) is one of the most commonly
ordered clinical laboratory tests. A CBC measures the RBCs, Hgb,
hematocrit (Hct), platelets, mean cell volume (MCV), mean cell
hemoglobin concentration (MCHC), mean platelet volume (MPV),
and total WBCs. CBC with differential will measure all the values
mentioned earlier; in addition, it will also measure the number of
each type of WBCs. An abbreviated method of noting hematologic
parameters in clinical practice is noted in the following figure.
Red Blood Cells (Erythrocytes)
Reference Range: Males, 4.3–5.9 × 106/μL or 4.3–5.9 × 1012/L
Reference Range: Females, 3.5–5 × 106/μL or 3.5–5 × 1012/L
Erythrocytes or RBCs are produced in the bone marrow, released
into the peripheral blood, circulated for ~120 days, and cleared by
the reticuloendothelial system. The primary function of RBCs is to
transport oxygen linked to Hgb from the lungs to tissues. The
concentration of RBCs in the blood can be measured to detect
anemia, calculate RBC indices, or calculate the Hct. Hct and Hgb
concentrations are generally used to monitor quantitative changes in
RBCs.
Hematocrit
Reference Range: Males, 40.7%–50.3% or 0.4–0.503
Reference Range: Females, 36%–44.6% or 0.36–0.446
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(Eq. 2-9)
(Eq. 2-10)
Hct (packed cell volume) is the percentage of RBCs to the total
blood volume and is determined by centrifuging a capillary tube of
whole blood and comparing the height of the settled RBCs to the
height of the column of whole blood. A decrease in Hct may result
from bleeding, the bone marrow suppressant effects of drugs,
chronic diseases, genetic alterations in RBC morphology, or
hemolysis. An increase in Hct may result from hemoconcentration,
polycythemia vera, or polycythemia secondary to chronic hypoxia.
Hemoglobin
Reference Range: Males, 13.8–17.5 g/dL or 138–175 g/L
Reference Range: Females, 12.1–15.3 g/dL or 121–153 g/L
Hgb is the major oxygen-carrying compound contained in RBCs.
Therefore, total Hgb concentration primarily depends on the number
of RBCs in the blood sample. As mentioned with Hct, medical
conditions that impact the number of RBCs will also affect Hgb
concentration. As discussed previously, glycated Hgb (A1c) is a
related test used to monitor diabetes mellitus.
Red Blood Cell Indices
RBC indices (also known as Wintrobe indices) are useful in the
classification of anemias. These indices include the MCV, the mean
cell hemoglobin (MCH), and the MCHC. These indices are
calculated in Eqs. 2-9 to 2-11:
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(Eq. 2-11)
MEAN CELL VOLUME
The MCV detects changes in cell size. A decreased MCV indicates a
microcytic cell, which can result from iron deficiency anemia or
anemia of chronic inflammation. A large MCV indicates a macrocytic
cell, which can be caused by a vitamin B12 or folic acid deficiency.
Underlying disease states (eg, habitual alcohol ingestion, chronic
liver disease, anorexia nervosa, hypothyroidism, reticulocytosis, and
hematologic disorders) may also present with an elevated MCV
secondary to deficiencies in these vitamins.69 The MCV can be
normal in a patient with a “mixed” (microcytic and macrocytic)
anemia. Note that a direct assessment of a blood smear by a
microscopic examination is the gold standard for confirming RBC
size.
MEAN CELL HEMOGLOBIN
The MCHC is a more reliable index of RBC Hgb than is MCH. MCH
measures the weight of Hgb in the RBCs in a sample, and MCHC
measures the concentration of the RBCs contained within a sample.
In normochromic anemias, changes in the size of RBCs (MCV) are
associated with corresponding changes in the weight of Hgb (MCH),
but the concentration of Hgb (MCHC) remains normal. Changes in
the Hgb content of RBCs alter the color of these cells. Thus,
hypochromic refers to a decrease in RBC Hgb, reflected by reduced
MCHC, and may indicate iron deficiency anemia. Conversely,
hyperchromic RBCs have an elevated MCHC because of the
presence of greater amounts of Hgb. Hyperchromic cells are not
commonly encountered.
Reticulocytes
Reference Range: Adults, 0.5%–1.5% of RBCs or 0.005–0.015
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Reticulocytes are young, immature erythrocytes, and typically
comprise about 1% of the RBCs. The reticulocyte count measures
the percentage of these new cells in the circulating blood. An
increase in the number of reticulocytes implies that an increased
number of erythrocytes are being released into the blood in response
to a stimulus. Reticulocyte count is a good indicator of bone marrow
activity because it represents a recent production. Because
erythrocytes regenerate rapidly, reticulocytosis can be noted within 3
to 5 days of hemolysis or after a hemorrhagic episode. Appropriate
treatment of anemias caused by iron, vitamin B12, or folic acid
deficiencies should result in an increased reticulocyte count. Caution
must be exercised in the interpretation of reticulocyte counts.
Changes in the number of RBCs will result in proportional changes in
the reticulocyte count because the latter is reported as a percentage
of the number of RBCs.
Erythrocyte Sedimentation Rate
Reference Range: 0–30 mm/hour
The ESR is the rate (expressed in mm/hour) at which erythrocytes
settle to the bottom of a test tube through the forces of gravity and in
response to fibrinogen levels in the blood. The ESR is a nonspecific
value and may be increased abnormally in acute and chronic
inflammatory processes, acute and chronic infections, neoplasms,
infarction, tissue necrosis, rheumatoid-collagen disease,
dysproteinemias, nephritis, and pregnancy. However, ESR can also
be affected by changes not related to the inflammation (eg, change
in erythrocyte size, shape, or number). Laboratory technique can
also affect the sedimentation rate substantially. Because many
factors can enhance the settling rate of RBCs, moderate to marked
elevation of the ESR merely indicates an inflammatory component to
a disease state. An increased ESR in the setting of a normal
physical examination is usually transient and is rarely the harbinger
of serious occult disease.
70
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