The influence of anticoagulant preservatives, time and posture in renin measurements and estimation of aldosterone to renin ratio (ARR)
KM Al-Hashmi, JJ Morton, HL Elliott & JM Connell
Background: The aldosterone to renin ratio (ARR) is a useful screening test for primary aldosteronism. Renin can be measured as activity or concentration; the accuracy of the ARR depends on these measurements and the factors that may affect them. We carried out two studies; the first assessed the correlation between plasma renin concentration (PRC) and renin activity (PRA) and the agreement between them using two anticoagulants (EDTA and lithium heparin). The second evaluated the effects of time and posture on renin and aldosterone. Both studies were approved by the hospital ethics committee.
Methods & results: 105 blood samples were collected from hypertensive patients. There was a positive correlation between PRC and PRA in samples stored using EDTA (r = 0.84, P< 0.001) and in LH (r = 0.85, P< 0.001). However, there were very wide 95% limits of agreement when the anticoagulants were compared (0.53 to1.7) for PRC and (0.55 to1.8) for PRA. Secondly, 15 healthy male volunteers were studied. After 30 minutes ambulation, subjects were seated, and samples were collected at 5, 10,30,60,90 and 120 minutes. PRC, PRA and aldosterone fell rapidly for the first 30 minutes and then more slowly thereafter. The difference with time was statistically significant (ANOVA P< 0.05 for PRC, PRA and aldosterone). However, there was no significant difference with time in ARR (P = 0.76 and P = 0.21 using PRC and PRA respectively).
Conclusion: both PRA and PRC can be used to measure renin in order to calculate the ARR. However, samples stored in LH may provide results that are less reliable. For an accurate ARR, measurements should ideally be taken after 30 minutes rest.