Spot Urine Potassium-Creatinine Ratio Is a Good Alternative Marker for 24-Hour Urine Potassium in Differential Diagnosis of Hypokalemia

BACKGROUND: Twenty-four-hour urine potassium (24-HUK) is a traditionally-used index to examine whether hypokalemia is due to renal potassium (K) loss in its differential diagnosis, but its accuracy is often compromised due to interference factors. The aim of this study was to evaluate whether the spot urine K-creatinine (Cr) ratio (SU(K/Cr)) can be used as a substitute for 24-HUK.
MATERIAL AND METHODS: Seventy hypokalemic patients who submitted both 24-hour and spot urine samples for clinical tests were recruited. Among them, 40 patients were found to have hypokalemia due to renal K loss. Thirty subjects were diagnosed with extra-renal hypokalemia. Thirty subjects with normal serum K levels were enrolled as controls. Before K supplement treatment in hypokalemia patients, spot urine samples were collected. A 24-hour urine sample and serum sample were also collected on the same day.
RESULTS: Both SU(K/Cr) and 24-HUK were significantly different between the patients with renal and extra-renal K loss (Non-renal group) (P=0.005 and P<0.001). Moreover, there were significant positive correlations between 24-HUK and SU(K/Cr) in the renal group (Renal group) (r=0.84, P=0.001) and non-renal group (r=0.42, P=0.02), as well as the controls with normal serum K (Control group) (r=0.59, P=0.001). Furthermore, through ROC curve analysis, 3.35 mmol/mmol was found to be the optimum cutoff value of SU(K/Cr) for renal K loss.
CONCLUSIONS: SU(K/Cr) may be a valuable marker to distinguish between renal and extra-renal K loss. Studies with larger sample sizes are needed to further confirm a specific cutoff value of SU(K/Cr).

Keywords: Creatinine, Hypokalemia, Kidney