From Denise Soileau, Helena Laboratories: It is my understanding that calibration verification is not required for prothrombin time. Is AMR and CRR? If so how do you suggest this be accomplished?
Hello, Denise, and thank you for your question. I’ve had some help from Larry Brace PhD, Edward Hospital, and Dave McGlasson MS, MLS, Wilford Hall USAF Hospital. It is true that we don’t usually do local calibration and calibration verification for the prothrombin time (PT), as PT reagents are calibrated to the international reference preparation (IRP) thromboplastin by the manufacturer. The calibration is reflected in the international sensitivity index (ISI) which is reset within local coagulometer circuitry when the reagent vial is scanned. The US FDA, in their wisdom, has never cleared a general set of PT calibrators, although Instrumentation Laboratory, Inc, does provide a set of local calibrators for their HemosIL PT reagent.
The analytically measurable range (AMR) is the range within which the assay provides accuracy, and may be slightly more restricted than the range of linearity. The clinically reportable range (CRR) in clinical chemistry applications is much wider than the AMR, as it includes the potential for diluting samples whose initial results fall outside the limits of the AMR. However, when measuring the PT, dilution is discouraged, as it distorts the plasma matrix, consequently for the PT, the CRR is usually the same as the AMR. I’ve not seen published AMRs or CRRs for the PT, however, standard coagulometers typically shut off at 110 seconds, providing an automatic upper limit.
Also, please take a look at McGlasson DL, Romick BG, Rubal BJ. Comparison of a chromogenic factor X assay with INR for monitoring oral anticoagulation therapy. Blood Coag Fibrinolys 2008;19:513–17, as it provides a graphic illustration of the sensitivity of the PT compared to the chromogenic factor X assay. The chart illustrates the poor discrimination of the PT when the international normalized ratio exceeds 5. Geo.