AN EASIER ALTERNATIVE TO ORTHOGONAL REGRESSION FOR CALCULATION OF INTERNATIONAL SENSITIVITY INDEXES

Aims-To evaluate an easier alternative method to orthogonal regression analysis for calculating International Sensitivity Indexes (ISI). Met hods-ISI for 18 reagents were estimated from reference and test reagen t prothrombin times using plasma from 60 stabilised patients undergoin g anticoagulation therapy and 20 normal subjects. ISI were also derive d for 12 systems (instrument/reagent combination) using lyophilised pl asma calibrants. Orthogonal regression and the easier alternative log ratio method were evaluated by comparing resultant International Norma lised Ratios (INR) for 58 patients using two test systems. Results-For the reagent calibrations, the differences in the two methods for the sensitivity slopes were very small. For the system calibrations, slope differences were still of little clinical importance. Parallel observ ations for INR on the 58 patients confirmed that the bias introduced b y easier log ratio derivation of ISI was small and of minor clinical i mportance, although the bias increases for high INR. Conclusions-The e asier method for ISI determination is a useful alternative to orthogon al regression analysis, particularly when computer assistance is not a vailable, for checking for gross errors in computer computation and fo r use when calculating ISI from INR calibrants.