In vitro sensitivity of different activated partial thromboplastin time reagents to mild clotting factor deficiencies

Introduction: Activated partial thromboplastin time (aPTT) is a routine clotting assay that is widely used to globally screen for coagulation abnormalities. It is commonly admitted that a prolonged test result, may trigger the need for specific assays to be performed, particularly factor measurement. However, the sensitivity of aPTT reagents to deficiencies of clotting factors varies. Methods: We evaluated, according to the recommendation of the CLSI H47-A2 guideline, the responsiveness to single factor levels of five aPTT reagents by using factor-deficient plasmas spiked with a calibration plasma to produce individual factor activities ranging from <1 to similar to 100 Unit (U)/dL. Test results were expressed as the sample-to-control ratio, the latter was defined as the clotting time obtained in the calibration plasma containing similar to 100 U/dL factor activity. The factor activity producing a prolongation of aPTT above the upper limit of its specific normal range (in ratio) was assigned as the factor responsiveness in U/dL to that reagent. Results: Responsiveness ranged from 34 to 47 U/dL to FVIII: C, from 18 to 57 U/dL to FIX, from 38 to 52 U/dL to FXI, from 29 to 50 U/dL to FXII, from 40 and 59 U/dL to FV, from 7.5 to 49 U/dL to FX, and from 9.1 to 10.5 U/dL to FII. Conclusions: These results suggest that the sensitivity of the tested aPTT reagents to single factor deficiency is highly variable. Moreover, for one given aPTT reagent, its sensitivity was very different depending on the deficient factor. This must be considered when analyzing clinical materials.