A 2-year-old post-neurosurgery develops polyuria (urine output 8 mL/kg/hr), urine specific gravity 1.003, serum Na 152 mEq/L, and serum osmolality 310 mOsm/kg. Which finding best differentiates central DI from nephrogenic DI in this child?
- A Rise in urine osmolality >50% after DDAVP administration ✓
- B Urine osmolality >600 mOsm/kg after water deprivation test
- C Serum copeptin level <2.9 pmol/L at baseline
- D Urine osmolality remains <150 mOsm/kg despite hypertonic saline infusion
Explanation
The DDAVP (desmopressin) response test differentiates central from nephrogenic DI. In central DI, the posterior pituitary fails to secrete ADH; exogenous DDAVP causes >50% rise in urine osmolality (response preserved). In nephrogenic DI, renal V2 receptors are non-functional; DDAVP produces minimal or no urine concentration. Water deprivation alone cannot differentiate (both fail to concentrate). Copeptin is a biomarker of AVP release — low in central DI — but the DDAVP response test is the classical clinical differentiator used in children. Hypertonic saline infusion is a provocative test for AVP secretion, not differentiation.
Reference: Ghai Essential Pediatrics, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.