Analysis index

10 ml spontaneous urine

and contemporaneous taking of

1 ml serum or plasma

> 12 % SIADH (after exclusion of other causes, see below)

< 8 % decreased effective arterial blood volume (fluid deficit)

Especially when taking diuretics, a differentiation of the cause of hypotonic hyponatremia (serum osmolality < 275 mosmol/kg) with a urine osmolality > 200 mosmol/kg and a urine-sodium value of >30 mmol/l is difficult. The diuretic-induced inhibition of tubular sodium resorption leads to a false increased sodium excretion, which can lead to a urine-sodium > 30 mmol/l and thus in up to 30 % of cases fakes the diagnosis of SIADH (syndrome of inadequate ADH-secretion), which with 40 % is the most common cause of hyponatremia. As uric acid transporters are only present in the proximal tubule, diuretics however work on the distal tubule or the Henle’s loop, the fractional uric acid excretion is suitable to support in the differential diagnosis even if under diuretic therapy.

Fractional uric acid excretion =

Urine-uric acid (mg/dl) x serum-creatinine (mg/dl)      x 100
Serum-uric acid (mg/dl) x urine-creatinine (mg/dl)

A fractional uric acid excretion of < 8 % is indicative for a decreased effective arterial blood volume (fluid deficit). However, fractional uric acid excretion of > 12 % after exclusion of having taken a thiazide-diuretic, adrenocortical weakness, renal tubular acidosis and cerebral salt-loss syndrome indicate a SIADH.