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Short synacthen test interpretation — SCE Endocrinology MCQ

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HardEndocrine emergencies/dynamic testsShort synacthen test interpretationSCE Endocrinology

During a specialist hypertonic-saline copeptin test for polyuria, plasma sodium reaches 148 mmol/L and copeptin is 3.8 pmol/L. Baseline anterior pituitary function is normal. Which interpretation is correct?

Educational content. Not a substitute for clinical judgement or local policy.

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Correct answer: A — Central AVP deficiency is supported because stimulated copeptin is at or below 4.9 pmol/L

Explanation lettering: E = shown as A · C = shown as B · A = shown as C · B = shown as E

E is correct. Once plasma sodium is raised to at least 147 mmol/L under close supervision, stimulated copeptin at or below 4.9 pmol/L supports central arginine-vasopressin deficiency, whereas a higher value favours primary polydipsia. A detectable low concentration does not exclude partial central deficiency. Nephrogenic AVP resistance typically produces high endogenous AVP/copeptin in response to hyperosmolality. The test carries hypernatraemia risk and belongs in an experienced unit with protocolised monitoring.

Reference: https://www.nejm.org/doi/full/10.1056/NEJMoa1803760