Blood collection tubes with protease inhibitors to prevent falsely high renin concentrations due to cryoactivation

Exposure of plasma samples to low temperatures (between -5 and + 4 °C) leads to conversion of prorenin to its open state, which is then converted to renin by plasma proteases. This process, known as cryoactivation, results in falsely elevated renin concentrations. This study examined the effects of storage temperature, protease inhibitors, and 2-hour incubation at 37 °C on renin cryoactivation.

Blood samples (n = 24) were collected in four types of tubes: K₂EDTA, aprotinin, trypsin inhibitor, and protease inhibitor cocktail. Renin concentrations (IDS-i10) were measured at baseline and after storage at + 4 °C (one week), -20 °C (two weeks), and -80 °C (two weeks). Samples stored at -20 °C were later incubated at 37 °C for two hours and reanalyzed. Aldosterone concentrations were measured (LC-MS/MS) and aldosterone to renin ratios (ARR) were calculated.

Renin concentrations remained stable at -80 °C but increased significantly at + 4 °C (18-50 %) and -20 °C (61-500 %). ARR changes were minimal at -80 °C (-8 pmol/mU) but largest at -20 °C (-263 pmol/mU). Renin concentrations returned to baseline after a 2-hour incubation at 37 °C only in trypsin inhibitor and protease inhibitor cocktail tubes.

Our study shows more cryoactivation of renin in plasma aliquots stored at -20 °C compared to storage at + 4 °C. Cryoactivation of prorenin in trypsin inhibitor and protease inhibitor cocktail tubes was reversed by a 2-hours incubation at 37 °C, which may be an alternative procedure for laboratories that are not able to store renin samples in -80 °C freezers.