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396 BASIC SKILLS IN INTERPRETING LABORATORY DATA
https://t.me/med1917
ACKNOWLEDGMENTS
e contribution of material written by James B. Groce III,
Julie B. Lemus, and Sheila M. Allen in previous editions of this
book is gratefully acknowledged.
LEARNING POINTS
1. What is the INR in relation to PT?
ANSWER: PT results are not standardized as various reagent
changes.
2. How can the d-dimer be used for diagnosis of VTE?
ANSWER:
done. The
tion; if the
-
tion of anticoagulation.
3. Which la boratory tests may be used to monitor DOACs?
ANSWER:
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PubMed
CHAPTER 17 • HEmATology: Blood CoAgulATion TEsTs 399
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QUICKVIEW | Platelet Count
PARAMETER DESCRIPTION COMMENTS
Common reference ranges
Adults
Critical value
>800,000 or <20,000 μL (>800 × 10
μ× 10
Inherent activity
Location
Production
Storage Two-thirds found in circulation,
Causes of abnormal values
High
Infection
9
9
<20 × 109
and other tissues
one-third found in spleen
Low
Severe B
12
Table 17-2
Table 17-3
Drugs
Signs and symptoms
High
Low
in patients with severe
After event, time to...
Initial elevation
Causes of spurious results
μL
9
(50 × 10
× 109
these instances
Hct <20 or >50%
400 BASIC SKILLS IN INTERPRETING LABORATORY DATA
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QUICKVIEW | PT and INR
PARAMETER DESCRIPTION COMMENTS
Common reference ranges
Adults PT 10–13 sec
Children
Critical value
Inherent activity
Location
Production Coagulation factors produced in liver
Storage
PT <16 sec
PT >15 sec
>5 is
has the shortest half-life and thus is
patient is on warfarin and depending
on indication for warfarin; usual
therapeutic ranges are either 2–3 or
2.5–3.5
Unless on warfarin
Causes of abnormal values
High Diseases: liver disease
Drug: warfarin
Low Table
Signs and symptoms
High
Low
antagonist
After event, time to...
Initial elevation 6–12 hr
Causes of spurious results
Additional info Target levels depend on indication
Table 17-8
17-8
increases
for warfarin
CHAPTER 17 • HEmATology: Blood CoAgulATion TEsTs 401
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QUICKVIEW | aPTT
PARAMETER DESCRIPTION COMMENTS
Common reference ranges
Adults 25–35 sec
Critical value
Inherent activity
Location
Production Coagulation factors produced in liver
Storage Two-thirds found in circulation, one-
Causes of abnormal values
High
Low
>
reagent
third found in spleen
Signs and symptoms
High
Low
After event, time to...
Initial elevation 6–12 hr
Causes of spurious results
instances
Additional info
institution
402 BASIC SKILLS IN INTERPRETING LABORATORY DATA
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QUICKVIEW | anti-Xa
PARAMETER DESCRIPTION COMMENTS
Common reference ranges
Adults
UFH
Inherent activity Used to establish therapeutic heparin
Location
Production Coagulation factors produced in liver
Storage
Causes of abnormal values
High
Poor renal function
preparation used
Low
Signs and symptoms
High
Low
After event, time to...
Initial elevation
Causes of spurious results
Additional info
DOAC
inhibitors
18
https://t.me/med1917
Infectious Diseases: Bacteria
Sharon M. Erdman, Rodrigo M. Burgos,
and Keith A. Rodvold
OBJECTIVES
After completing this chapter, the
reader should be able to
•
Discuss the common tests used by
the microbiology laboratory for the
•
List the types of clinical specimens
that may be submitted for Gram
stain and culture
•
Describe the process of staining
and culturing a clinical specimen for
bacteria, including the time required
to obtain a result from either
method; discuss the clinical utility
of the information obtained from a
Gram stain and a culture result
•
Identify bacteria according to
Gram stain result (gram-positive
versus gram-negative), morphology
(cocci versus bacilli), and growth
characteristics (aerobic versus
anaerobic)
•
sites of the human body where
and those that are usually sterile; list
bacteria that are considered normal
•
Describe the most common causative
pathogens based on infection type or
anatomic site of infection
•
Describe the common methods
used for antimicrobial susceptibility
testing including technique, type
of result, clinical implications,
and limitations of each method;
demonstrate the ability to
appropriately use susceptibility
information when choosing an
antimicrobial regimen for a patient
(continued on page 404)
DOI 10.37573/9781585286423.018
e assessment, diagnosis, and treatment of a patient with an infection may appear
to be an overwhelming task to some clinicians. is may be partly due to the nonspecic presentation of many infectious processes; the continuously changing taxonomy,
diagnostic procedures, and antimicrobial susceptibility patterns of infecting organisms; and the continuous introduction of new antimicrobials to the existing large
collection of anti-infective agents. is chapter focuses on the laboratory tests used
for the diagnosis of the most common infections caused by bacteria.
Information regarding white blood cells (WBCs) and their role in infection is
discussed in Chapter16; laboratory tests used in the diagnosis of viral hepatitis,
Helicobacter pylori gastrointestinal (GI) infection, and Clostridioides dicile pseudomembranous colitis are addressed in Chapter15; and lab tests used for the diagnosis of viral, fungal, and mycobacterial infections are discussed in Chapter19. Lastly,
information regarding the clinical utility of the erythrocyte sedimentation rate (ESR)
and C-reactive protein (CRP) as they relate to inammatory diseases and infections
are also addressed in Chapter20.
BACTERIA
Bacteria are small, unicellular, prokaryotic organisms that contain a cell wall but
lack a well-dened nucleus. ey are a diverse group of microorganisms that exist in
dierent shapes and morphologies with varying rates of pathogenicity. Bacteria are
a common cause of infection in both the community and hospital setting, and can
cause infection in patients with normal or suppressed immune systems. Bacteria must
be considered potential causative pathogens in any patient presenting with signs and
symptoms of infection.
Specimen Collection and Identification of Bacteria
Several factors should be considered when choosing an appropriate antimicrobial
regimen for the treatment of infection, including patient characteristics (eg, immune
status, age, end-organ function, comorbidities, drug allergies, and severity of illness),
drug characteristics (eg, spectrum of activity, pharmacokinetics, penetration to the
site of infection, and proven clinical ecacy), and infection characteristics (eg, site/
type of infection [suspected or known] and potential causative organism(s)). erefore,
appropriate diagnosis is a key factor in selecting appropriate empiric and directed
antibiotic therapy for the treatment of an infection. In the case of a suspected infection, appropriate culture specimens should be obtained for laboratory testing from
the suspected site of infection before antibiotics are initiated, if possible, in an attempt
to isolate and identify the causative pathogen. Special attention should be placed on
specimen collection and timely transport to the laboratory because the accuracy of the
results is limited by the quality and integrity of the submitted specimen.
lists common biologic specimens that may be submitted to the microbiology laboratory for bacteriologic analysis.
When a specimen from the suspected site of infection is submitted to the microbiology laboratory, several microbiologic tests are performed to aid in the identication
of the infecting bacteria. e most common laboratory tests used for the identication of bacteria include direct microscopic examination using specialized stains
1-4
1-4
Table18-1
403
404 BASIC SKILLS IN INTERPRETING LABORATORY DATA
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OBJECTIVES
•
50,
90, minimum inhibitory concentration susceptibility
breakpoint, and minimum bactericidal concentration
•
Describe the information that is used to construct a
cumulative antibiogram; discuss the clinical utility of
the cumulative antibiogram when choosing empiric
antibiotic therapy for the treatment of a patient’s
infection
•
List the laboratory tests that may be performed for
the diagnosis of infections due to miscellaneous or
uncommon organisms such as Borrelia burgdorferi,
Treponema pallidum, and Legionella pneumophila
•
Describe the clinical utility of laboratory tests
routinely performed for the diagnosis of infection in
meningitis is suspected, (2) respiratory secretions
when lower respiratory tract infections are suspected,
(3) urine, prostatic secretions, or genital secretions
when a genitourinary tract infection is suspected, and
(eg, Gram stain, uorescent stains such as acridine orange or
auramine-rhodamine stains) and bacterial culture techniques to
foster growth of the microorganism. Once bacteria grow in culture, additional tests are then performed to identify the infecting
organism and determine susceptibility of the bacteria to various
antimicrobial agents.
TABLE 18-1.
Abscess, lesion, wound, pustule: swab or aspirate
Blood
Bone marrow
Body uids: amniotic, abdominal, bile, pericardial,
peritoneal, pleural, or synovial by needle aspiration
Bone: biopsy of infected area
CSF: by lumbar puncture or directly from shunt (eg,
ventriculoperitoneal shunt)
Cutaneous: hair or nail clippings, skin scrapings, aspiration
of leading edge of skin infection; biopsy
Ear:
specimen by swab or biopsy
Eye: conjunctival swab, corneal scrapings, aqueous or
Foreign bodies: intravenous catheter tip by roll plate
method; prosthetic heart valve, prosthetic joint material,
intrauterine device, etc.
GI tract:
biopsy for H pylori, rectal swab, stool cultures, stool
specimen for C difcile
Genital tract: cervical, endometrial, urethral, vaginal, or
prostatic secretions; ulcer biopsy
Respiratory tract:
pharyngeal or nasopharyngeal swab, sinus aspirate
Tissue: biopsy
Urine: clean-catch midstream, straight-catheterized,
suprapubic aspirate
Gram Stain
Gram stain is the most common staining method used for the
microscopic examination of bacteria and is most appropriate for
the evaluation of body uids (eg, cerebrospinal uid [CSF], pleural, synovial, etc.), respiratory tract secretions, and wound/abscess
swabs or aspirates.4 e Gram stain classies bacteria into one of
two groups, gram-positive or gram-negative, based on their reaction to an established series of dyes and decolorizers. e dierence in stain uptake between gram-positive and gram-negative
bacteria is primarily due to dierences in their bacterial cell wall
composition and permeability.
5-8
Although the Gram stain does
not provide an exact identication of the infecting organism (eg,
Klebsiella pneumoniae versus Serratia marcescens), it does pro-
vide rapid (within minutes) preliminary information about the
potential infecting organism that can be used to guide empiric
antibiotic therapy while waiting for culture results, which may
take 24 to 48 hours or more. e Gram stain is useful for characterizing most clinically relevant bacteria but is unable to detect
intracellular bacteria (eg, Chlamydia), bacteria without cell walls
(eg, Mycoplasma), and organisms that are too small to be visualized with light microscopy (eg, spirochetes).
6-8
e current Gram stain methodology is a slight modication
late 19th century.
5-8
e Gram stain procedure involves several
staining and rinsing steps that can be performed within a few
Source: References 1–4.
minutes. e rst step involves applying, drying, and heat-xing
a thin smear of a biological specimen to a clean glass slide. Once
the slide has cooled, it is then rinsed with crystal or gentian violet (a purple dye) followed by Gram’s iodine, decolorized with
an ethanol or acetone rinse, and then counterstained with safranin (apink or red dye), with a gentle tap water rinse performed
between each of these steps. e slide is then blotted dry and
examined under a microscope using the oil immersion lens. If
bacteria are present, they are examined for stain uptake, morphology (round = coccus, rod = bacillus), and organization (eg, pairs,
clusters). Gram-positive bacteria stain purple due to retention of
the crystal violet-iodine complex in their cell walls, whereas gramnegative bacteria stain red because they do not retain crystal violet
and are counterstained by safranin.
5,6,9
e results of the Gram
stain (eg, gram-positive cocci in pairs or gram-negative rods) may
provide information about the possible infecting organism before
the culture results become available and, in some situations, may
also be used to guide empiric antibiotic therapy. Table18-2 lists
the most likely bacteria based on Gram stain results.
10,11
Once
antibiotic regimen can be de-escalated, if necessary, to target the
infecting bacteria (directed therapy).
CHAPTER 18 • InfECTIous DIsEAsEs: BACTERIA 405
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TABLE 18-2.
GRAM STAIN RESULT LIKELY BACTERIAL PATHOGEN
Gram-positive (stain purple)
Gram-positive cocci in clusters Staphylococcus spp.
Coagulase-positive: S aureus
Coagulase-negative: S epidermidis, S hominis, S saprophyticus, S haemolyticus,
S lugdunensis, etc.
Gram-positive cocci in pairs Streptococcus pneumoniae
Gram-positive cocci in chains
Gram-positive cocci in pairs and chains Enterococcus spp. (E faecalis, E faecium, E durans, E gallinarum, E avium,
Gram-positive bacilli
Nonspore-forming Corynebacterium spp. (C diphtheriae, C jeikeium, C striatum, etc.)
Bacillus spp. (B anthracis, B cereus, etc.)
Actinomyces spp. (A israelii)
Viridans (α-hemolytic) streptococci (S milleri, S mutans, S salivarius, S mitis, etc.)
Group (β-hemolytic) streptococci (S pyogenes [group A], S agalactiae
Finegoldia magna, Peptostreptococcus spp., Peptoniphilus spp., Parvimonas micra
E casseliavus, E rafnosus)
Lactobacillus spp.
Listeria monocytogenes
Cutibacterium spp.
Clostridium spp. (C perfringens, C tetani)
Clostridioides spp. (C difcile)
Streptomyces spp.
Erysipelothrix rhusiopathiae
Nocardia spp. (N asteroides)
Gram-negative (stain red)
Gram-negative cocci Neisseria spp. (N gonorrhoeae, N meningitidis, etc.)
Veillonella spp. (V parvula)
Gram-negative coccobacilli Haemo philus spp. (H inuenzae, H parainuenzae, H ducreyi, etc.)
Moraxella catarrhalis
Gram-negative bacilli
Lactose-fermenting Aeromonas hydrophila
Citrobacter spp. (C freundii, C koseri)
Enterobacter spp. (E cloacae, E asburiae, E gergoviae, E taylorae)
Escherichia coli
Klebsiella spp. (K pneumoniae, K oxytoca, K aerogenes)
Pasteurella multocida
Vibrio cholerae
Nonlactose-fermenting Acinetobacter spp.
Alcaligenes spp.
Burkholderia cepacia
Morganella morganii
Proteus spp. (P mirabilis, P vulgaris)
Pseudomonas spp. (P aeruginosa, P putida, P uorescens)
Salmonella spp. (S typhi, S paratyphi, S enteritidis, S typh imurium)
Serratia marcescens
Shigella spp. (S dysenteriae, S sonnei)
Stenotrophomonas maltoph ilia
(continued)
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