Uric acid, anion gap and urea concentration in the diagnostic approach to hyponatremia.

We analyzed the serum anion gap (AG = sodium plus potassium minus chloride plus bicarbonate, N = 11-21 mEq/l), serum uric acid and urea concentrations in hyponatremia of various origins. We found that characteristic chemical patterns emerged in association with different hypotonic states: Low uric acid concentration was typically observed in the SIADH and in hyponatremia related to hypopituitarism. The same observation was also frequently noted in hyponatremia secondary to diuretics or to polydypsia. In the SIADH, we observed a decrease in the AG but to a greater extent (-26%) than one would expect from the simple dilutional effect (-16%). Fifty percent of the patients presented an AG lower than 11 mEq/l. In patients with diuretic-related hyponatremia, one group presented an hypouricemia and a low AG as in SIADH (reflecting volume expansion), in the other group the AG was normal or increased as was uric acid concentration (reflecting volume depletion). In adrenocorticotropin deficiency, hyponatremia was typically associated with a low bicarbonate concentration, a normal AG and hypouricemia. In polydypsic patients with hyponatremia, the AG was usually normal or increased despite sometimes very low sodium levels. Uric acid levels were highly variable, most often decreased. We also noted in these patients that the serum urea levels were correlated with urine osmolality (R = +0.8; p < 0.001), and in 40% of them we observed very low blood urea concentration (0.5-2 mmol/l) at the admission time. In hyponatremia related to cardiac failure or cirrhosis, the AG was usually normal despite mild hypoproteinemia.