Local calibration of International Normalised Ratio improves between laboratory agreement: Results from the UK National External Quality Assessment Scheme

In the present study we have performed local calibration of International N ormalised Ratio (INR) measurement systems in a large series of laboratories . We assigned INRs to five lyophilised plasma calibrants, one prepared from normal plasma and four using plasma from warfarinised patients, using diff erent International Reference Preparations for Thromboplastin. These five c alibrants. and two lyophilised test plasmas were analysed by 349 centres us ing 60 different thromboplastin instrument combinations. Plasma calibrants were assigned INRs using the WHO reference thromboplastin RBT-90 or the Europe;tn reference thromboplastin CRM I 149R. Each particip ating centre determined PTs of the calibrants with their local system. Thes e PTs were then used to construct a local calibration graph relating PT to INR. The PTs of test plasmas were converted directly into INR using the loc al calibration model and into INR using the conventional method. The overal l medians of conventionally derived INRs of two test plasmas analysed in 34 9 centres were 2.50 and 3.10, compared to 2.47 and 3.04 after local calibra tion where RBT-90 was employed to assign INRs to calibrants. Use of CRM 149 R to assign INRs to calibrants led to a significant (p <0.0001) increase in INR to 2.7 and 3.36 respectively. When results were grouped according to t he thromboplastin employed agreement between results with different reagent s was improved by local calibration. There was a significant reduction (p < 0.01) in the spread of results in different centres as indicated by a reduc tion in coefficient of variation.