Prothrombin time (PT) and activated partial thromboplastin time (APTT), popularized as a routine assay for screening
blood coagulation disorders and monitoring
anticoagulant therapy, still involve some issues regarding standardization. In this lecture, we present propositions to resolve these problems in respective laboratory. Although international normalized ratio (INR) calculated by international sensitivity index (ISI) of PT
reagent seems to improve discrepancy of sensitivity between
reagents, local calibration of sensitivity of PT
reagent in respective laboratories (local SI) is reasonable to make INR/ISI system more useful. However, local calibration of
reagent is not easy by WHO recommended method in a small size laboratory. By using AK calibrant (IMMUNO AG), one of calibration plasma for INR, we investigated its possibility to calibrate local SI in four different
reagents, compared with the recommended methodology. The results led the following process to determine
reagent and calibrate local SI for practical use of INR/ISI system. (a) Use PT
reagent of which ISI is close to 1.0 if possible, and utilize manufacture's ISI as is for INR. (b) Select PT
reagent labeled specific ISI for an instrument as the same as used in the lab., and use the manufacture's ISI as is, if impossible to choose small ISI
reagent. (c) If use a
reagent of which ISI is close to 2.0 and shown no specific ISI for used detector, adjustment of local SI by commercial calibration plasma is recommended when unavailable warfarinized patient plasma. In APTT, we attempted to evaluate sensitivity between five different APTT
reagents with a patient model by
hemophilia A plasma contained various FVIII: C. This model reflected difference of sensitivity between
reagents in results. Because standardization of APTT is not improved in this point, certification of APTT pattern in each laboratories with patient models is required for not only monitoring of heparinization, but also screening of typical coagulation disorders such as
hemophilia and
von Willebrand disease.