Diagnostic Implications of Renin Reactivity in Confirmatory Tests: A Comparative Study of Direct Renin Concentration and Plasma Renin Activity in Primary Aldosteronism.

Objective: Renin measurement is a key to diagnosing primary aldosteronism (PA). However, it remains unclear whether plasma renin activity (PRA) or direct renin concentration (DRC), which are based on different principles, offer superior performance. This study aimed to compare their diagnostic potential in PA. Methods: This cross-sectional observational study was conducted at a single institution, and enrolled consecutive patients hospitalised for PA diagnosis. PRA and DRC were measured for the same samples during the captopril challenge, saline infusion, and furosemide upright tests. We evaluated renin responsiveness, defined as renin changes from baseline in each test, and the relationship between PRA and DRC. Results: Seventy-two patients were classified into PA and non-PA groups. The patients in the PA group were further classified into unilateral and bilateral PA groups using adrenal vein sampling. The renin responsiveness positivity was significantly higher for DRC than for PRA in the PA group across all tests. The DRC/PRA ratio did not differ significantly between PA and non-PA groups, while the DRC/PRA ratio was significantly lower in the unilateral PA group than in the bilateral PA group. The correlation between PRA and DRC was weak in the PA group, especially in the low-renin range. Conclusions: Renin changes were more consistently detected with DRC than with PRA in the low-renin range. The DRC/PRA ratio was lower in unilateral PA than in bilateral PA, suggesting potential utility for recognising unilateral PA, although this finding should be considered exploratory.