IS THE ‘SODIUM INDEX’ A USEFUL WAY OF EXPRESSING CLINICAL PLASMA RENIN,ANGIOTENSIN AND ALDOSTERONE VALUES?

In normal subjects taking variously high, normal or low sodium diets, while potassium intake was maintained within the normal range, highly significant inverse relationships were demonstrated between 24 h urinary sodium output and the concurrent plasma concentrations of renin, angiotensin II and aldosterone. With linear coordinates, these relationships were described by rectangular hyperbolae. With logarithmic plots the relationships became rectilinear. When 24 h urinary sodium output was 75 mmol or higher, there was no worthwhile advantage in relating renin to sodium excretion; plasma angiotensin II and aldosterone, however, were significantly related inversely to urinary sodium in this range. When 24 h urinary sodium output was below 10 mmol, plasma renin, angjotensin II and aldosterone all varied over wide but elevated ranges and were not significantly related to urinary sodium excretion rates. We conclude that whereas estimations of urinary sodium may be of value in revealing latent aberrations of sodium intake, there is no distinct advantage in relating measurements of renin or angiotensin II to urinary sodium output if the diet is known to be within broad ‘normal’ limits. This should simplify the relevant blood sampling procedure under both ward and outpatient conditions. If plasma renin, angiotensin II and aldosterone are to be related to concurrent urinary sodium output, logarithmic, rather than linear, co-ordinates, are appropriate.