Minimum lyophilized plasma requirement for ISI calibration

The minimum requirement of lyophilized plasma samples for a reliable International Sensitivity Index (ISI) was assessed by calibrations based on reducing numbers from a maximum of 60 depleted and 20 plasma samples from patients receiving coumarin using a manual technique and low ISI thromboplastin. The probability of achieving an international normalized ratio (INR) result within a clinically important range of 20% deviation has been assessed at European Concerted Action on Anticoagulation (ECAA) national laboratories in calibrations of ECAA reference thromboplastin against the certified prothrombin time (PT) with a reference thromboplastin. With conventional orthogonal regression using a certified PT and linear regression using certified INR, deviations and the coefficient of variation of the calibration slope increased with reduced numbers. The INR deviations became marked when the number of abnormal plasma samples was reduced to fewer than 20. Calibrations of 3 abnormal plasma samples and 1 lyophilized normal plasma sample gave a high incidence of deviations greater than 10% with an INR of 3.0. The study demonstrates that with both methods of analysis, an optimum minimum number of lyophilized plasma samples is needed for a reliable local ISI.