Ömer Özcan1,2, Jacquelien Hillebrand1,2, Carla Beertsen2, Wendy den Elzen1, Annemieke Heijboer 1,2,
(1) Amsterdam UMC location University of Amsterdam, Department of Laboratory Medicine, Amsterdam, The Netherlands. (2) Amsterdam UMC location University of Amsterdam, Endocrine Laboratory, Department of Laboratory Medicine, Amsterdam, The Netherlands.
Background and aim:
Cryoactivation of renin is a commonly known phenomenon caused by cold exposure triggering the conversion of prorenin to renin by plasma proteases. This leads to falsely high plasma renin concentrations. As cryoactivation may occur between +4°C and even –20°C leading to major increases in plasma renin concentrations, correct storage of plasma samples for renin analyses is challenging. Therefore, we aimed to study, first, the possibility of preventing cryoactivation by using blood collection tubes containing protease inhibitors, and second, the reversibility of cryoactivation by incubation of plasma samples at 37°C.
Methods:
From 24 adult volunteers, blood samples were collected in (I) K3EDTA, (II) aprotinin, (III) trypsin inhibitor and (IV) protease inhibitor cocktail containing tubes. After centrifugation at room temperature, plasma samples were aliquoted and measured: 1) Immediately, after storage for 2) one week at +4°C, 3) two weeks at -20°C, or 4) two weeks at -80°C. Subsequently, aliquots were incubated at 37°C for 2-hours and measured again. Renin concentration measurements were performed using the i10 (IDS).
Results:
Plasma renin concentrations in aliquots stored at -80°C were similar to freshly measured aliquots (median differences, –7 to 0%). Aliquots stored at +4°C showed 19-50% (median) higher renin concentrations compared to fresh aliquots. Aliquots stored at-20°C showed higher renin levels than fresh measurements (median 61-190% higher for tubes containing protease inhibitors and 500% higher for normal K3EDTA tubes). The increased concentrations after storage at –20°C were back to fresh measurement levels after 2-hour incubation at 37°C when using trypsin inhibitor and protease inhibitor cocktail tubes but not when using EDTA or aprotinin tubes.
Discussion:
Our study shows more cryoactivation of renin in plasma aliquots stored at -20°C compared to storage at 4°C. Cryoactivation of renin in trypsin inhibitor and protease inhibitor cocktail tubes could be reversed by a 2-hours incubation at 37°C, which may be an alternative procedure for laboratories that are not able to measure plasma renin in fresh samples or do not have -80°C freezers available to prevent cryoactivation.