A liquid chromatography tandem mass spectrometry assay for plasma renin activity using online solid-phase extraction

BACKGROUND: The plasma renin activity (PRA) assay measures the ability of renin to generate angiotensin I (AngI) from angiotensinogen. It is used to monitor mineralocorticoid therapy and to screen hypertensive individuals for primary aldosteronism (PA). METHODS: Samples were incubated in the presence of protease inhibitors for 6.5 and 24 h. The reaction was stopped by the addition of 2% ammonium hydroxide. AngI was then quantified by liquid chromatography tandem mass spectrometry using online solid-phase extraction (XLC-MS/MS). RESULTS: This method requires a sample volume of 50 μL and has an inter-assay precision <14% across the working range. A 6.5-h incubation gave a lower limit of quantification (LLOQ) of 0.3 nmol/L/h and this can be reduced to 0.08 nmol/L/h using a 24-h incubation. Comparison to a radioimmunoassay revealed excellent correlation (r(2) = 0.98), but a 37% negative bias. We also found that renin is stable in whole blood for up to 24 h at room temperature. In contrast, storage at 4°C should be avoided as prorenin cryoactivation can affect the PRA result in some patient groups. CONCLUSIONS: We have developed and fully validated a semi-automated XLC-MS/MS method for the measurement of PRA. In addition, a reference range specific to this assay has been defined. We have also demonstrated that renin is stable for up to 24 h at room temperature. This will enable this assay to be extended to samples taken in primary care, potentially increasing the number of hypertensive patients who can be screened for PA.