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1328 Coombs’ Test Cortisol

Pearls obtained from microscopic examination of See discussion of various red blood cell
Know how the laboratory you use determines stained blood smears remain the most accurate morphologic abnormalities (p. 1374).
VetBooks.ir whether blood smears are examined routinely. left shifts and toxic changes in neutrophils, and AUTHOR: Stephen D. Gaunt, DVM, PhD, DACVP
method for differentiating leukocytes, detecting
differential leukocyte concentrations and
EDITOR: Lois Roth-Johnson, DVM, PhD, DACVP
for recognition of shape changes.
Although automated hematology analyzers
provide three- or five-part differential leukocyte
counts as part of a CBC, the differential counts

Coombs’ Test

Definition Next Diagnostic Steps to Consider bound immune complexes that were eluted
Detects immunoglobulin or complement bound if Levels are High during washing, or detachment of immu-
to the surface of erythrocytes to aid in the See p. 60. noglobulin or C3 from sample aging.
diagnosis of primary or secondary immune-
mediated hemolytic anemia Causes of Abnormally Low Levels Specimen Collection and Handling
Look for non–immune-mediated causes of Whole blood collected in EDTA (lavender top
Synonym hemolysis (see p. 59), but false-negative tests tube). Store at 8°C. Test as soon as possible.
Direct antiglobulin test (DAT) occur, so negative test cannot rule out IMHA.
Relative Cost: $$
Physiology Important Interspecies Differences
Patient erythrocytes are washed with saline to Species-specific reagents are required to perform Pearls
remove nonspecific bound surface proteins. this test. • Coombs’ testing is not standardized and
Species-specific Coombs’ reagent is added that is often unnecessary given other clinical
includes polyvalent antisera that will bind to Drug Effects pathology data.
IgM, IgG, and complement (C3) bound to the Treatment with glucocorticoids or other • Negative Coombs’ testing does not rule out
surface of the patient’s erythrocytes, resulting immunosuppressive drugs may result in a IMHA.
in agglutination. false-negative test. Prior transfusions can cause • Positive test results do not confirm primary
positive result. IMHA, and secondary causes, such as infec-
Reference Interval tious disease or neoplasia, should be ruled
Reported as “no agglutination” or “agglutination Lab Artifacts out.
observed” at a specific titer. Typically incubated • Prozone reaction is common when the ratio • Coombs’ test provides no information
to body temperature, but some laboratories of antibody to antigen is inappropriate, regarding immune-mediated disease other
report room temperature results as well (i.e., resulting in false-positives. than IMHA.
cold agglutinins) • Hemolysis suggests that there was formation
of immune complexes between antisera and AUTHOR: Erin N. Burton, DVM, MS, DACVP
Causes of Abnormally High Levels surface-bound immunoglobulins EDITOR: Lois Roth-Johnson, DVM, PhD, DACVP
Primary or secondary immune-mediated hemo- • False-negative may occur with low surface-
lytic anemia (IMHA), prior blood transfusion bound immunoglobulins and/or C3, weakly

Cortisol

Definition test, or high-dose dexamethasone suppression attributable to either overproduction of
Cortisol is a glucocorticoid that has far- test. ACTH or direct overproduction of cortisol
reaching effects on gluconeogenesis, lipolysis, from a functional adrenal cortical neoplasm.
immune responses, and water balance, among Physiology Atrophy or destruction of the adrenal cortices
other things. Cortisol measurement is useful ACTH release from the pituitary promotes results in hypoadrenocorticism, including
in diagnosing patients with hypoadrenocorti- adrenocortical secretion of cortisol. Increasing hypocortisolemia.
cism and hyperadrenocorticism. Baseline value cortisol concentration in the blood will normally
alone is insufficient for diagnosis and instead inhibit further ACTH production, facilitating Reference Interval
is compared with another value (example, homeostasis. Cortisol concentration fluctu- Varies by laboratory, and commonly
cortisol to ACTH ratio or urine cortisol to ates throughout the day and can be affected expressed as either mcg/dL or nmol/L. Inter-
creatinine ratio) or measured at multiple by disease states, other stressors, and drug pret in light of established reference interval
time points during the ACTH stimulation administration. Pathologic hypercortisolemia for species by given laboratory and testing
test, low-dose dexamethasone suppression is the main feature of hyperadrenocorticism, methodology.

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