Urate (Urine)

Diagnostic Use

For hyponatraemia:

In healthy euvolemic subjects, Fractional Excretion of Urate (FEurate) is approximately 5-10%. In differentiating hyponatraemia induced by syndrome of inappropriate secretion of antidiuretic hormone (SIADH) versus hypovolaemic (e.g. by diuretics), hypouricaemia (<0.24mmol/L) and increased FEurate (>10-12% in adult) can be used as markers for SIADH (hypouricaemia is not as sensitive or specific as raised FEurate).

FEurate (Spot urine) is calculated as:

   Urine urate (mmol/L) x  Plasma creatinine (mmol/L) x 100% /  Plasma urate (mmol/L) x Urine creatinine (mmol/L). 

Urine sodium concentration (UNa) demonstrated a reasonably good diagnostic performance for SIADH but only in those not on diuretics.  It also cannot differentiate SIADH from cerebral/renal salt wasting. On the other hand, FEurate still performs well in those on Frusemide diuretics. Of note is that there has been suggestions that its diagnostic performance is not as high in those on Thiazide diuretics especially with euvolaemic phenotype. FEurate <8% has high negative predictive value in excluding SIADH. FEurate <4% is suggestive of volume depletion, addison’s disease or pre-renal azotemia e.g. from cirrhosis, heart failure or nephrosis.

A raised FEurate is not entirely specific for SIADH. Other conditions like use of uricosurics e.g. Probenecid or Losartan, Hodgkins disease, cerebral/renal salt wasting can also raise FEurate.

In SIADH, fluid restriction with correction of hyponatraemia usually result in simultaneous correction of hypouricaemia and the FEurate. In contrast, in cerebral/renal salt wasting the elevated FE urate remain unchanged despite normalisation of plasma sodium. That can be a distinguishing feature for these two conditions with overlapping biochemical features but require opposite ways of clinical management.

For Gout and Renal calculi investigation:

Assessing renal urate excretion is generally unnecessary in managing patients with gout. It is mostly in the domain of specialists’ request. Indications may include consideration to use uricosuric agents, investigate patients with renal calculi, gout onset at age under 25 or family history of young onset gout.

Reference Intervals

24hr urine urate excretion: 1.5 – 4.4 mmol/day (adult male and female)

Spot urine urate over creatinine ratio (mmol/mmol):

Male (≥18 years): 0.07 – 0.40

Female (≥18 years): 0.06 – 0.50

Children: