Photodynamic treatment of pooled coumarin plasma for external quality assessment of the pro thrombin time

Aims-To determine the conditions of photodynamic inactivation of vesicular stomatitis virus (VSV) added to pooled coumarin plasma and the effects of t he photodynamic treatment on the prothrombin times and international normal ised ratio (INR) in a Netherlands national external quality assessment sche me. Methods-Pooled coumarin plasma samples were illuminated with visible light in the presence of 1 mu M methylene blue. Inactivation conditions for VSV i n pooled coumarin plasma were determined using an end point dilution assay. Plasma illuminated for 20 minutes was mixed with red blood cells and maile d to participants of the Netherlands external quality assessment (EQA) sche me. Prothrombin times and INRs were determined with various thromboplastin reagents. Results-Photodynamic treatment using 1 mu M methylene blue and 700 W/m(2) c aused 4.7 log inactivation of VSV in pooled coumarin plasma. Fibrinogen and coagulation factors II, V, VII, and X were decreased slightly by the treat ment. These conditions caused prolongation of the prothrombin time in EQA s urveys. The magnitude of the effect was different for various thromboplasti n reagents. The increase of the INR was negligible when measured with the T hrombotest reagent. With other reagents, an approximately 5-16% increase of the INR was observed. Interlaboratory variation of the INR was not affecte d by photodynamic treatment. Conclusions-Photodynamic treatment of pooled coumarin plasma is very effect ive for the inactivation of some enveloped viruses such as VSV, but has onl y a limited effect on the prothrombin time and INR. Photodynamic treatment can be used to improve the viral safety of coumarin plasma for EQA of the p rothrombin time and INR.