RESUMO
Factors that influence the physico-chemical conditions of plasma (e.g. pH, dilution, freezing, storage) and thereby the stability of tPA and PAI-1 activities, have been studied and optimized using a solid-phase fibrin-tPA activity assay. Optimal recovery of tPA activity was at a pH of 6.8 +/- 0.2, while at the pHs usually found in thawed plasma, i.e. pH 7.6-8.2, the activity was lower and showed great variability. Free tPA activity was tested in undiluted plasma, while plasma diluted 1:20 was used to recover maximal tPA activity. The corrected value for the diluted plasma and the value for the euglobulin suspensions were similar. In both cases the pH optimum was 7.4. PAI activity levels were tested in undiluted plasma and showed no variations after venous occlusion. Our results indicate that the in vitro determination of tPA activity is directly related to the pH of thawed plasma and not to the freezing procedure or the temperature of storage. Therefore, thawed plasma should be tested at a pH giving the maximal recovery of tPA activity in a particular assay method.
