Hyponatraemia is a common electrolyte disorder of which the syndrome of inappropriate antidiuretic hormone secretion (SIADH) is a frequent cause. A contraindication however in the diagnosis of SIADH is the use of drugs that affect water balance, i.e. diuretics. In the August issue of JCEM, Fenske et al. compared the diagnostic utility of urine sodium and fractional excretion (FE) of sodium against other volume-related parameters – namely the FE-urea and FE-urate, and the serum urate concentration, to aid in the diagnosis of SIADH in patients on diuretic therapy. These markers have been proposed because diuretic-related interference is not expected. Their data showed that in patients on diuretics, the use of urine sodium or FE-sodium resulted in a pronounced loss of diagnostic accuracy. The FE-urate exhibited the best overall performance to diagnose SIADH in patients on diuretics (area under curve, 0.96; 0.91–1.00; P = 0.05). A cut-off value of 12% appeared to be optimal to confirm the diagnosis of SIADH with a sensitivity of 86%, and a specificity and positive predictive value of 100%. The utility of this marker is not superior however to urine sodium in patients not on diuretics. Furthermore, the diagnostic value of FE-UA may be limited in cirrhotic patients and in patients with cerebral salt-wasting syndrome, where an elevated FE-urate (>12%) has also been reported. Therefore, caution must be applied before a diagnosis of SIADH is made in these instances. The use of medications, such as uricosuric drugs (i.e. probenecid) and losartan, an angiotensin II receptor antagonist should also be considered. The authors conclude that the use of FE-urate is a useful tool, with a high degree of accuracy for the identification of SIADH in hyponatraemic patients on diuretics and may avoid the need to withdraw the use of diuretics in the diagnostic workup.