Assessment of serum sodium to urinary sodium divided by (serum potassium)² to urinary potassium as a screening tool for primary aldosteronism

Abstract

The SUSPPUP ratio [(serum sodium/urinary sodium)/(serum potassium²/urinary potassium)] has been proposed as a marker to screen for primary aldosteronism (PA). The original study found an area under the receiver operating characteristic (ROC) curve of 0·90 to detect PA; the sensitivity was 89% and specificity 86% for a ratio over 5·3 L mmol⁻¹. Patients attending a hypertension unit between 2001 and 2006 and for who renin and aldosterone measurements and concomitant serum and urinary biochemistry data were available were included if diagnosed with PA (n = 449) or essential hypertension (n = 2209). We compared the diagnostic value of the SUSPPUP ratio and of serum potassium in the whole population, in patients without interfering drugs and in patients with lateralized PA. The area under the ROC curve was significantly worse for the SUSPPUP ratio than for serum potassium in all groups: 0·72 vs. 0·76 in the whole population; 0·73 vs. 0·78 without interfering drugs; 0·76 vs. 0·82 for patients with lateralized PA. In the whole population, sensitivity was 71% for a SUSPPUP ratio ≥ 5·3 L mmol⁻¹ and serum potassium < 3·7 mmol L⁻¹, but specificity of the SUSPPUP ratio was significantly worse (61% vs. 69%). Using low serum potassium and/or high SUSPPUP ratio increased the sensitivity to 87% but decreased the specificity to 47%. The SUSPPUP ratio was outperformed by serum potassium as a screening tool for PA in this large validation sample. Its value as an adjunct to serum potassium is questionable because of the low specificity of their combination.