Identifying Different Causes of Hyponatremia With Fractional Excretion of Uric Acid

There is controversy over the prevalence of the syndrome of inappropriate antidiuretic hormone secretion (SIADH) and cerebral or renal salt wasting (RSW), 2 syndromes with identical common clinical and laboratory parameters but different therapies. The traditional approach to the hyponatremic patient relies on volume assessment, but there are limitations to this method.

We used an algorithm that relies on fractional excretion of urate (FEurate) to evaluate patients with hyponatremia and present 4 illustrative cases.

Overall, 2 patients had increased FEurate [normal: 4-11%], as is seen in SIADH and RSW. A diagnosis of SIADH was made in 1 patient by correcting the hyponatremia with 1.5% saline and observing a characteristic normalization of an elevated FEurate that is characteristic of SIADH as compared to FEurate being persistently increased in RSW. A patient with T-cell lymphoma had symmetrical leg edema due to lymphomatous obstruction of the inferior vena cava, postural hypotension, pleural effusion, ascites, decreased cardiac output and urine sodium level of 10mmol/L. Saline-induced excretion of dilute urines and undetectable plasma antidiuretic hormone were consistent with RSW. Furosemide, given for presumed heart failure, induced a profound diuresis that required large volumes of fluid resuscitation. A normal FEurate identified a reset osmostat in a transplant patient with a slowly developing pneumocystis carinii pneumonia. A volume-depleted hyponatremic patient with Addison׳s disease had a low FEurate of 1.4%.

These illustrative cases suggest that an approach to hyponatremia using FEurate may be a useful alternative to traditional volume-based approaches.