A SIMPLIFIED STATISTICAL-METHOD FOR LOCAL INR USING LINEAR-REGRESSION

A simplified method of International Normalized Ratio (INR) derivation using linear regression of certified INR plotted against local prothr ombin time (PT) results has been compared with INR from conventional o rthogonal regression. Linear regression assumes error only with the lo cal PT results whereas orthogonal regression assumes error with both r eference and local results. The reliability of local INR derivation us ing lyophilized plasmas has been assessed in a collaborative study. IN R from conventional fresh plasma International Sensitivity Index (ISI) calibrations have been compared with INR from calibrations with two t ypes of lyophilized plasma, artificially depleted and coumarin. Althou gh calibration slopes differed with the two types of analysis and the different lyophilized plasmas, both gave reasonable approximations to fresh plasma ISI calibrations. With orthogonal regression the overall percentage INR deviation was 5.25% with the artificially depleted plas mas and 6.85% for the results with lyophilized coumarins. With the lin ear regression, deviation was 8.40% for the artificially depleted plas mas and 5.05% for coumarin-treated patients' lyophilized plasmas. The simpler regression method appears to be worthy of further study as the present report has demonstrated that if the calibrant plasmas are acc urately certified with the thromboplastin International Reference Plas ma (IRP) results approximate to the conventionally determined INR usin g the manual PT technique. Coagulometers require further assessment.