Serum to urinary sodium concentration ratio is an estimate of plasma renin activity in congestive heart failure

We investigated the relationship between plasma renin activity (PRA) and serum ([sNa(+)]) and urinary ([uNa(+)]) sodium concentrations in 124 congestive heart failure (CHF) patients (II-IV NYHA class) and 20 healthy subjects. According to PRA (> or <3 ng ml(-1) h(-1)) and [sNa(+)] (> or <135 mEq l(-1)), patients were classified as Group A (normal PRA and normal [sNa(+)], n=39), Group B (increased PRA and normal [sNa(+)], n=62) and Group C (low [sNa(+)], n=23). Measurements were performed at rest and, in 26 cases, after extracorporeal ultrafiltration (UF). At rest, [sNa(+)] and [uNa(+)], and their difference ([sNa(+)]-[uNa(+)]), were linearly correlated with PRA, but the values did not allow differentiation of control subjects from patients or differentiation of patients with from those without renin-angiotensin system (RAS) activation. Conversely, the [sNa(+)]/[uNa(+)] ratio showed the best correlation with PRA (r=0.79, P<0.0001). UF-induced PRA changes were linearly correlated with [sNa(+)]/[uNa(+)] ratio changes (r=0.67, P=0.002), but not with those of [sNa(+)], [uNa(+)] and [sNa(+)]-[uNa(+)]. In CHF, the [sNa(+)]/[uNa(+)] ratio best correlates with PRA and reflects the basal activity as well as the rapid changes (as those induced by UF) of the RAS. Therefore, it can be considered a strong and easily available marker of PRA.