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High-Dose Dexamethasone Suppression Test Howell-Jolly Bodies 1353

bore needle to transfer or introduce blood • Ensure prompt separation of serum from the • Avoid exposing sample to temperature
specimens into Vacutainer tubes to avoid clot and proper storage of sample (refrigera- extremes.
VetBooks.ir • Avoid excess negative pressure when drawing • Avoid delay in sample analysis or delayed AUTHOR: Ruanna E. Gossett, DVM, PhD, DACVP
tion; avoid freezing).
excess turbulence/foaming and resultant
hemolysis.
EDITOR: Lois Roth-Johnson, DVM, PhD, DACVP
transport of samples to the laboratory for
blood into syringe and excessive agitation of
the sample in the tube. analysis.

High-Dose Dexamethasone Suppression Test

Definition very rarely in patients with ectopic ACTH • EDTA and icterus also interfere with
The high-dose dexamethasone suppression production from a nonpituitary neoplasm. assay.
test (HDDST) is a dynamic test of the
hypothalamic-pituitary-adrenal axis used to Next Diagnostic Steps to Consider Specimen Collection and Handling
differentiate between pituitary-dependent if Levels Are High Three blood samples for cortisol measurement
hyperadrenocorticism (PDH) and the presence Abdominal imaging to elucidate whether are collected. For each, collect blood into plain
of a functional adrenal cortical neoplasm in a one adrenal gland is enlarged (supportive of red top tubes, allow to clot, centrifuge, and
patient known to have hyperadrenocorticism. functional adrenal cortical neoplasm), or if transfer serum to a separate tube clearly labeled
Baseline and post-dexamethasone blood samples there is bilateral adrenomegaly (supportive with the sample ID/time. Ship samples frozen
are collected for cortisol measurement. of PDH). Consider measurement of endogenous or refrigerated, on ice.
ACTH concentration if confusion still exists. • Baseline cortisol sample drawn in morning
Synonym See pp. 34, 485, and 487. • Administer 0.1 mg/kg dexamethasone IV
HDDST (dogs) or 1 mg/kg dexamethasone IV (cats).
Causes of Abnormally Low Levels • Collect additional blood sample at 4 hours
Physiology Adequate suppression during HDDST indicates and again at 8 hours post-dexamethasone
Administration of dexamethasone should pituitary micro- or macroadenoma causing PDH. administration.
suppress pituitary ACTH production and thus
adrenocortical cortisol production. Functional Next Diagnostic Steps to Consider Relative Cost: $$
adrenal neoplasms and some (≈35%) pituitary if Levels Are Low
neoplasms will continue to autonomously Suppression of cortisol production with the Pearls
secrete cortisol and ACTH, respectively. HDDST in a patient known to have hyper- • Due to ready accessibility of advanced
adrenocorticism confirms PDH. imaging, HDDST is now seldom used to
Reference Interval discriminate between adrenal and pituitary-
Varies by laboratory. Examples of guidelines for Important Interspecies Differences dependent hyperadrenocorticism. Laboratory Tests
“adequate” suppression of cortisol levels post- Dose of dexamethasone differs between species. • HDDST is not used for the diagnosis
dexamethasone administration are < 1.4 mcg/dL of hyperadrenocorticism and should be
or suppression by more than half of baseline Drug Effects performed only after confirming that
cortisol concentration. Cortisol measured as Cortisol assays may cross-react with pharma- hyperadrenocorticism is present.
either mcg/dL (common unit) or nmol/L (SI cologic steroids administered within 24 hours, • Though suppression of cortisol concentration
unit). including prednisolone, 11-deoxycortisol, confirms PDH, failure to suppress does not
prednisone, and others. discriminate between functional adrenal
Causes of Abnormally High Levels cortical neoplasia and PDH.
Failure of HDDST to suppress cortisol often Lab Artifacts
indicates the presence of functional adrenal • Do not use serum separator tubes for blood AUTHOR: Angela Royal, MS, DVM, DACVP
EDITOR: Lois Roth-Johnson, DVM, PhD, DACVP
cortical neoplasia. Failure to suppress cortisol collection; gel from serum separator tubes
can also be seen with some (≈35%) PDH and can interfere with cortisol measurement.

Howell-Jolly Bodies

Definition nuclei. Howell-Jolly bodies represent nuclear in cats than dogs. Only reported on blood smear
Retained erythrocyte nuclear fragments. Basophilic fragments that have separated from the nucleus review if excessive
intraerythrocytic inclusions that are generally of the developing erythrocyte. They are removed
single, spherical, and ≈0.5 to 1 mcm in diameter by the spleen. Causes of Abnormally High Levels
Regenerative anemia, decreased splenic
Physiology Reference Interval function/splenectomy, hereditary macrocytosis
During development in the bone marrow, Howell-Jolly bodies are occasionally found in of poodles, myelodysplastic syndrome, or drugs
mammalian erythroid progenitor cells contain the erythrocytes of healthy animals, more often (e.g., glucocorticoids, chemotherapeutics)

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