Sample processing time but not storage time affects complement activation markers C4a, C4d, C3a, iC3b, Bb, C5a, and sC5b-9 levels in EDTA-plasma of individuals at clinical high-risk for psychosis

The complement system is an important part of the innate immune system and plays a key role in inflammatory processes. Concentrations of complement activation fragments in plasma are markers of systemic activation and have been found to be altered in a wide range of diseases. Some plasma activation marker levels can be influenced by sample processing and storage time. We quantified seven complement activation markers (C4a, C4d, C3a, iC3b, Bb, C5a, and sC5b-9 (TCC)) in EDTA-plasma as part of a multi-centre clinical study analysing complement activation in individuals with clinical high-risk (CHR) for psychosis compared with healthy controls. Samples had been collected, processed, and subsequently stored at -80°C over a period of 9.5–13.6 years, according to a standard operating protocol (SOP). Complement activation markers were quantified using commercially available and standardised enzyme-linked immunosorbent assays (ELISA). In a post hoc analysis of variables affecting the analyses we investigated the impact of EDTA-to-freezer processing time (<1–7.35 hours) and freezer storage time (9.5–13.6 years). EDTA-to-freezer processing time moderately correlated positively with C4a, C3a, iC3b and sC5b-9 levels. Storage time at -80°C was not significantly correlated with any complement activation marker. This study provides valuable insight into the impact of sample processing and long-term sample storage in complement activation marker studies. The results suggest that storage time in -80°C is not a confounding factor affecting non-specific complement activation in EDTA-plasma. Sample-processing time does moderately affect the levels of some complement activation markers. This should be considered as a co-variate when analysing complement activation marker levels. Further, the impact may vary for healthy or clinical samples where immune activation is part of the pathology. These findings are important when planning large-scale clinical studies that include quantification of complement components and its activation fragments as biomarkers. It supports the collection of EDTA-plasma and fast sample processing to be included into a study standard operating procedure.