Table 1.

The results of the water load test for our patient using a protocol adapted from a previous publication (10).

(min)	Urine output (mL)	Venous gas Na (mmol/L)	Lab Na (mmol/L)	K (mmol/L)	Glu (mmol/L)	Urea (mmol/L)	Creatinine (µmol/L)	Serum osmolality (mosmol/kg)	Urine osmolality (mosmol/kg)	Co-peptin (pmol/L)
0		136.0	134	4.4	5.1	5.9	48	280	710	<1.3
60	300	131.1	128		4.9	4.3	43	272
120	300	129.1	128		4.9	4.2	43	264		<1.3
180	200	128.1	129	3.8	4.8	3.9	37	266
240	40	131.1	128	3.8	4.6	3.9	37	262		<1.3
360	100	130.5	128	3.9	4.6	3.9	39	267	495
09:00 h following morning	N/A	N/A	134	4.4	4.6	5.9	48	287	609

A water load based on the following calculation (20 mL/kg = 1508 mL) was performed. Within 60 min of the water load consumption, serum sodium dropped to 128 mmol/L from 134 mmol/L but co-peptin remained suppressed consistent with a diagnosis of nephrogenic SIAD. Furthermore, the patient failed to excrete the expected 70–80% (1056–1206 mL) of the water load volume within four hours, as indicated by the urine output (840 mL in 4 h).