Copeptin in the differential diagnosis of the polydipsia-polyuria syndrome--revisiting the direct and indirect water deprivation tests.

BACKGROUND: The water deprivation test (WDT) with direct or indirect measurement of plasma arginine vasopressin (AVP) is the method of choice for the differential diagnosis of the polydipsia-polyuria syndrome. In theory, direct measurement of AVP is highly attractive but is hampered by technical difficulties. OBJECTIVE: The aim of the study was to evaluate the utility of copeptin, a surrogate of AVP secretion, in the diagnostic work-up of the polyuria-polydipsia syndrome and to compare its performance with the current diagnostic standard. SETTING AND DESIGN: In two tertiary referral centers, 20 healthy subjects and 50 patients with polydipsia-polyuria syndrome underwent WDT with measurements of both plasma AVP and copeptin levels. The reference diagnosis was based on clinical information and treatment response. RESULTS: Twenty-two patients (44%) were diagnosed with primary polydipsia, 17 (34%) with partial central diabetes insipidus (DI), nine (18%) with complete central DI, and two (4%) with nephrogenic DI. The indirect WDT led to a correct diagnosis in 35 of 50 patients (70%). The direct WDT with AVP or copeptin measurement correctly diagnosed 23 patients (46%) or 36 patients (72%), respectively. Baseline copeptin values greater than 20 pmol/liter identified patients with nephrogenic DI, and concentrations below 2.6 pmol/liter indicated complete central DI. The ratio between Δ copeptin (0800 to 1600 h) and serum sodium concentration at 1600 h yielded optimal diagnostic accuracy, allowing us to also discern partial central DI from primary polydipsia (sensitivity 86%, and specificity 100%). CONCLUSION: Copeptin holds promise as a diagnostic tool in the polyuria-polydipsia syndrome, improving significantly the diagnostic accuracy of the direct WDT.

Value of fractional uric acid excretion in differential diagnosis of hyponatremic patients on diuretics.

BACKGROUND: The syndrome of inappropriate antidiuresis (SIAD) is the most frequent cause of hyponatremia. Its diagnosis requires decreased serum osmolality, inappropriately diluted urine (e.g. >100 mOsm/kg), clinical euvolemia, and a urinary sodium (Na) excretion (U-Na) more than 30 mmol/liter. However, in hyponatremic patients taking diuretics, this definition is unreliable due to the natriuretic effect of diuretics. Here, we examined the diagnostic potential of alternative laboratory measurements to diagnose SIAD, regardless of the use of diuretics. METHODS: A total of 86 consecutive hyponatremic patients (serum Na <130 mmol/liter) was classified based on their history, clinical evaluation, osmolality, and saline response to isotonic saline into a SIAD and a non-SIAD group. U-Na, serum urate concentration, and fractional excretion (FE) of Na, urea, and uric acid (UA) were measured in all subjects. The accuracy to diagnose SIAD was assessed using receiver operating characteristic analysis. RESULTS: A total of 31 patients (36%) had a diagnosis of SIAD, and 55 (64%) were classified as non-SIAD. There were 57 patients (68%) who were on diuretics (15 in the SIAD group, 42 in the non-SIAD group). In the absence of diuretic therapy, SIAD was accurately diagnosed using U-Na (area under the receiver operating characteristic curve 0.96; 0.92-1.02). However, in patients on diuretics, the diagnosis was unreliable (area under the curve 0.85; 0.73-0.97). There, FE-UA performed best compared with all other markers tested (area under the curve 0.96; 0.92-1.12), resulting in a positive predictive value of 100% if a cutoff value of 12% was used. CONCLUSION: FE-UA allows the diagnosis of SIAD with excellent specificity. Combining the information on U-Na and FE-UA leads to a very high diagnostic accuracy in hyponatremic patients with and without diuretic treatment.