Introduction: The ‘‘spot’’ urine sodium/potassium (Na/Ku) ratio is a convenient tool to identifysodium excretion < 78 mequiv./d. It has been established that this ratio is as useful and accurateas 24-h urine sodium (Nau24h), but no Latin American study has evaluated this issue. The aim ofthis study is to evaluate the accuracy of the Na/Kuratio and to compare it to Nau24h.Materials and methods: This cross-sectional study included 20 patients with decompensatedliver cirrhosis and ascites.Results: Mean age was 56.1 ± 11.8 years old, 70% were men and 60% presentedNau24h< 78 mequiv./d. Poor sodium excretion group presented higher MELD score (P = 0.002),creatinine (P = 0.029), AST (P = 0.027) and bilirubin (P = 0.013) levels, and also lower medianspot sodium urine (P = 0.001). There was a high positive correlation between Na/Kuratio andNau24h(r = 0.857; P < 0.001) and negative correlation with MELD (r = −0.498; P = 0.025) and serumcreatinine (r = −0.498; P = 0.025). The AUROC of Na/Kuin predicting Nau24h< 78 mequiv./d was0.948 ± 0.046 and the classical Na/Kuratio cut-off (≤1.0) showed PPV = 70%, NPV = 90%, accu-racy = 80%, sensitivity = 88% and specificity = 75%.Conclusion: Na/Ku≤ 1 is sensitive and specific, and substantially correlates with Nau24h, whichenables the use of this test in the routine evaluation of patients with liver cirrhosis decompen-sated in ascites.