Inhibitor Index in the Clot Waveform Analysis-Based Mixing Test Differentiates among Hemophilia A without and with Inhibitors, and Lupus Anticoagulant

Abstract

Background The mixing test is used to identify the pathway to follow-up testing and is also useful for the investigation of lupus anticoagulant (LA) positivity. “To completely correct” indicates clotting factor deficiency, while “to not correct” indicates the presence of a clotting factor inhibitor including LA. “Index of circulation anticoagulant” and/or “percent correction” is used to interpret the results of mixing studies, but it does not accurately differentiate factor inhibitors from LA. Aim To precisely differentiate hemophilia A (HA), HA with inhibitor (HA-inh), and LA using the clot waveform analysis (CWA)-based mixing test. Methods Plasma samples from HA, LA, and HA-inh including acquired HA were incubated with normal plasma in 9:1, 1:1, and 1:9 mix ratios. From activated partial thromboplastin time CWA at 0-minute (immediately) and 12-minute incubation, the ratios of CWA parameters at 12 minutes/0 minute (inhibitor index) were assessed. Results The inhibitor index values of CWA parameters obtained using the mixing test in a 1:1 ratio demonstrated a significant difference between HA-inh and LA but could not differentiate LA from HA-inh completely. Plasmas used for the mixing tests in 9:1 and 1:9 ratios were able to fully distinguish between HA-inh (>0.5 BU/mL) and LA. These indices significantly correlated with inhibitor titer below 40 BU/mL (r > 0.90), possibly estimating FVIII inhibitor titer from the inhibitor index. Plasmas in HA and LA could be distinguished by mixing in a 1:1 ratio at 0 minute (immediately). Conclusion The inhibitor index from CWA-based mixing tests with a 12-minute incubation could differentiate among HA, HA-inh, and LA quickly.

Advances in laboratory assessment of thrombosis and hemostasis

“…Beyond pharmacodynamic studies, CWA was a useful screening tool for hemophilia A [ 23 ]. Furthermore, FVIII specific inhibitors were easily differentiated from lupus anticoagulants when mixing APTT data was interpreted by CWA [ 24 ]. The most fundamental approach of CWA is probably the attempt to analyze the shape itself of the clotting curve.…”