Multicenter evaluation of lyophilized and deep-frozen plasmas for assignments of the International Normalized Ratio

The interlaboratory variation of the International Normalized Ratio (INR) i n various external quality assessment schemes is still relatively high. Thi s is partly caused by inaccuracy of manufacturers' stated International Sen sitivity Index (ISI) and/or local instrumentation effects, The interlaborat ory variation and accuracy of INR determinations may be improved by a local calibration procedure based on lyophilized plasmas with assigned INRs. The purpose of the present study was to determine INR values for different typ es of lyophilized plasmas to be used for local calibration. A total of 13 l yophilized plasmas tone normal, six from coumarin-treated patients, six art ificially depleted) were analyzed by 10 laboratories, each using five calib rated prothrombin time (PT) systems. INRs were calculated for each plasma u sing each laboratory's specific ISI and mean normal prothrombin time values . In the same way, five deep-frozen pooled plasmas from coumarin-treated pa tients were analyzed. There were significant INR differences for the lyophi lized plasmas between the prothrombin time systems, The differences were re latively small for the deep-frozen coumarin plasmas (CV 2.6-3.3%) and three lyophilized coumarin plasmas from one manufacturer (CV 3.7-4.8%). Importan t INR differences were observed for three lyophilized coumarin plasmas from another manufacturer (CV 9.5-14.1%) and several artificially depleted plas mas (CV 5.3-12.8%. The citrate concentrations in the artificially depleted plasmas were lower than those in the normal and coumarin plasmas. These dif ferences should be considered in the selection and certification of plasmas as calibrants for local calibration of PT systems. The lyophilized plasmas ' INR values obtained in the present study will be used for a field study o f local PT calibration to assess their efficacy.