RESUMO
OBJECTIVE: Protein S(PS) activity, especially PS-specific activity calculated by total PS activity (tPSAct) divided by total PS antigen (tPSAg), is important in the diagnosis of hereditary PS deficiency (PSD). The cleavage of PS at a thrombin-sensitive region (TSR) by proteases reduces the anticoagulant activity of PS. Therefore, we investigated the effect of sample processing and storage on tPSAct and PS cleavage. METHODS: Blood samples were collected from ten healthy subjects, and tPSAg and tPSAct were measured in whole blood or plasma stored at room temperature (RT) or 4°C. The cleaved PS was detected by western blotting, and the relationship between decreases in PS-specific activity and increase rates of cleaved PS was evaluated. Furthermore, the stability of tPSAg and tPSAct on the long-term storage of plasma was also evaluated. RESULTS: Both whole blood and plasma stored at RT and whole blood stored at 4°C showed decreased tPSAct (50-80%) after 24 hours (P<0.05). PS-specific activity levels negatively correlated with the increase rate of cleaved PS (r =-0.84, P<0.001). The tPSAct was decreased to 60% after three days in plasma stored at 4°C (P<0.05) but was stable for about one month when stored at -20°C or below. CONCLUSION: Inappropriate processing and storage result in falsely low PS-specific activity due to the cleavage of PS in the blood collection tubes, which may lead to misdiagnosis of PSD. Samples should be centrifuged immediately after collection, and the plasma should be frozen.
