Dexamethasone Suppression Test (low dose)

Screening test to confirm the diagnosis of Cushing’s disease/PPID (Horse)

Serum 2 x 0.5 ml or 3 x 0.5 ml

dog/cat/guinea pig

• First blood collection = baseline value
• Injection of dexamethasone i.m. or i.v. dog/guinea pig: 0,01 mg/kg bdw, cat: 0,1 mg/kg bdw
• Blood collection 4 hours post injection = 1st suppression value
• Blood collection 8 hours post injection = 2nd suppression value

• First blood collection = baseline value (blood sampling at around 4 – 6 pm)
• Injection of 2 mg/50 kg bdw of dexamethasone i.v.
• Blood sampling approx. 15 hours after administration of dexamethasone (at around 8 – 10 am) = 1st suppression value – may be omitted
• 2nd suppression value after approx. 18 – 20 hours (at around 10 am – 1 pm) – obligatory
• Because of the circadian rhythm, the indicated times of the day should be observed.

Dog, cat, guinea pig, horse

dog/cat

• Normal: baseline value in reference range or slightly elevated (due to stress), 4 hours post injection reduction by 50% or to < 10 ng/ml and 8 hours post injection to < 10 ng/ml. .
• Cushing’s disease: baseline value in reference range or elevated and one or both suppression values > 10 ng/ml.
• The additional blood collection 4 hours p.i. gives information on whether Cushing’s disease is pituitary or adrenal.
• Pituitary: baseline value in reference range or elevated, 4-hour value reduced by 50% or to < 10 ng/ml and 8-hour value > 10 ng/ml.
• Adrenal tumour: baseline value in reference range or elevated, no adequate reaction to the administration of dexamethasone after 4 and 8 hours.

• PPID: one or both suppression values > 10 ng/ml
• Cave: In late summer/autumn, healthy horses, too, possibly suppress insufficiently.

• Evaluation: adequate suppression and interpretation as in dogs, reference values not available.

• The test with examination of 2 suppression values (3 serum samples) can be found on the online submission forms and in the practice programmes under test number 1142.
• PPID (pituitary pars intermedia dysfunction), formerly called Cushing's disease in horses is caused by “pituitary adenoma” (hyperplasia of the pars intermedia). The hyperplastic cells have no cortisol receptors, that is why in PPID, the exogenous administration of dexamethasone does not suppress the endogenous secretion of corticoids as it does in healthy horses.