Pre-analytical and analytical considerations for the determination of plasma renin activity.

BACKGROUND: Primary hyperaldosteronism diagnosis is helped by assaying renin concentrations either by immunodetection (active renin) or by enzymatic reaction followed by angiotensin I immunoassay (plasma renin activity). We investigated the impact of pre-analytical and analytical conditions on a PRA assay.
METHODS: PRA was assayed using a commercial kit. Firstly, a retrospective analysis of PRA results from 1yr was performed. Secondly, the impact of pre-analytical temperature conveyance/storage of samples was tested (4 and 25 degrees C). Thirdly, 2 durations of enzymatic reaction 1.5 and 18h were tested.
RESULTS: Retrospectively, 9.5% samples displayed elevated "blanks": calculating PRA with or without "blank" subtraction elicited a difference of >10% in 1/3 samples. There was a significant decrease of PRA values in samples left at 25 degrees C vs 4 degrees C (50% "blank" increase). Finally, the longer the enzymatic reaction (1.5 vs 18h), the higher the production of angiotensin I and the better the PRA assay sensitivity.
CONCLUSION: We propose a 2 step procedure: a brief 1.5h enzymatic reaction time followed by a prolonged reaction (18h) only if PRA is <0.2ng mL(-1) h(-1). We would recommend performing "blank" samples for the brief enzymatic reaction time. Finally, conveyance/storage temperature does modify PRA results albeit in our hands this had little clinical impact.