Comparison of soluble thrombomodulin, von Willebrand factor, tPA/PAI-1 complex, and high-sensitivity CRP concentrations in serum, EDTA plasma, citrated plasma, and acidified citrated plasma (Stabilyte) stored at -70 degrees C for 8-11 years

The aim was to define the most suitable specimen collection tubes for measurements of soluble Thrombomodulin (sTM), von Willebrand factor (vWF), and tPA/PAI-1 complex concentrations, and in particular whether the strongly acidic citrate additive in Stabilyte plasma would give significantly improved long-term stability of any of these analytes. We measured these analytes in paired specimens from 34 subjects, sampled 8-11 years before analysis, in serum, EDTA plasma, citrated plasma, and acidified citrated plasma (Stabilyte). Results were evaluated by regression analysis and Bland-Altman plots. All associations were linear across a wide assay range. Soluble TM was found to be highly unstable in serum as well as in EDTA plasma and to some extent even in ordinary citrate plasma: acidified citrate plasma is necessary to preserve sTM immunoreactivity in long-term storage. For hsCRP the slopes were not significantly different from that predicted by the dilution effect (0.83-0.86) of the citrate additive and there was no appreciable intercept. vWF values were comparable in citrate and acidified citrate plasma but serum and EDTA plasma samples yielded lower than expected results. For tPA/PAI-1 complex, Stabilyte tubes gave systematically lower results than the other tubes, with serum and EDTA plasma scoring the highest values, suggesting that in vitro increase in complex levels takes places upon blood collection and/or storage. We conclude that Stabilyte plasma is the specimen collection tube of choice for biobank projects aiming to measure fibrinolytic factors as well as several other analytes in the clotting system, such as soluble thrombomodulin and von Willebrand factor, in addition to the inflammatory marker hs-CRP. Indeed, using acidified Stabilyte plasma as the single medium would substantially simplify sampling for many epidemiological studies.