Validation of current practice and a near patient testing method for oral-anticoagulant control in general practice

When oral anticoagulant control is monitored in general practice, venous blood samples are usually transported to a central laboratory for determination of international normalized ratio (INR). An alternative is near patient testing by a commercial method. In a rural general practice 27 km from a central haematology laboratory, whole blood samples were drawn from patients receiving oral anticoagulants and analysed by three methods: after centrifugation, plasma separated and frozen in liquid nitrogen, transported to the laboratory, thawed and immediately analysed (control); courier transport of citrated sample to the laboratory for analysis (routine); near patient testing of whole blood sample (NPT). Maximum temperature achieved and time to analysis for routine samples were recorded. 306 complete sets of data were obtained. Comparison between the routine method and the control method revealed acceptable agreement. On multiple regression analysis, maximum temperature achieved did not contribute to differences observed but time to analysis of over 5 hours did make a significant contribution. Comparison between the NPT method and control method showed acceptable agreement, with persistent under-recording by the NPT method. The routine method for INR determination was validated as robust and reproducible with the proviso that needle-to-analysis time should be kept below 5 hours. The NPT method was valid under conditions of normal general practice. Strict quality control of NPT methods is essential if performance is to be comparable with that of established methods.