The measurement of plasma renin activity is required in a number of clinical situations, in particular screening for primary aldosteronism (PA) and monitoring mineralocorticoid replacement therapy. PA is a treatable cause of hypertension and has an estimated prevalence of up to 20% amongst resistant hypertensives. Consequently, recent guidelines now recommend screening for PA in all patients groups with a high prevalence of PA. At present, the most reliable method of screening for PA is to calculate the plasma aldosterone to renin ratio (ARR), with a raised ARR being suggestive of PA.
The plasma renin activity (PRA) assay measures the ability of plasma renin to generate angiotensin I (Ang1) from endogenous angiotensinogen, with all established assays using an immunoassay to quantitate AngI. We have now developed and validated a method for the measurement of PRA which involves extraction and quantitation of Ang1 by online solid phase extraction-LCMS/MS. This method requires a sample volume of 50 μl and has an intra-assay precision <7% across the working range of the assay. A 6.5 h incubation step gave a LLOQ of 0.3 nmol/l per hour and this can be reduced to 80 pmol/l per hour using a 24 h incubation, allowing the measurement of very low activity samples. In comparison to the established immunoassays, this method will potentially improve assay specificity and will also reduce turnaround time, consumable costs, staff time, sample handling and sample volume requirement.
There is uncertainty in the literature regarding the stability of renin at room temperature and also the extent to which prorenin cryoactivation occurs. We have now studied the stability of PRA, with preliminary results suggesting that PRA is unaffected by storage of whole blood at room temperature for at least 24 h. This would enable us to analyse blood samples taken in primary care, potentially providing more effective screening of the hypertensive population.