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

BACKGROUND: 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. MATERIALS AND METHODS: 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. RESULTS: 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%. CONCLUSIONS: 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.