Urine electrolytes as markers of bulimia nervosa

Objective: The diagnosis of bulimia nervosa (BN) is often delayed because patients are frequently secretive about the illness. Prior work has examined several potential diagnostic markers, none of which has been both highly sensitive and specific. Little is known about the utility of urine electrolytes in detecting BN symptoms. Method: Seventy-seven women with BN and 53 control women participated in the study. Urine and serum electrolytes and urine phenolthalein screens were obtained. Subjects with BN also completed a self-report instrument (the Eating Behaviors IV) regarding vomiting during the week prior to assessment. Receiver operating characteristic analysis was used to examine the predictive abilities of urine and serum electrolytes. Results: Bulimic and control subjects differed significantly on most electrolyte measures. The ratio of urine sodium to urine chloride was the best predictor of bulimic behavior; selecting individuals with a ratio of >1.16 identified 51.5% of BN subjects with a 5% false-positive rate. Fractional excretion of sodium (FENA), urine anion gap (UAG), and serum potassium values were also predictive of BN but serum hypokalemia was not more common in BN than in control subjects (4.1% vs. 0%; p = .15). Vomiting frequency was correlated with an abnormal UAG (r² = .2231) but not FENA, nor serum potassium. Conclusion: The ratio of urine sodium to urine chloride is a useful predictor of bulimic behavior that appears to be more powerful in detecting BN than traditional screening measures such as serum hypokalemia.