Background: Emicizumab comprises a bispecific monoclonal antibody mimicking the function of FVIIIa by binding FIXa and FX leading to formation of FXa and has the advantage of a half-life of approx. 30 days versus 12hr for FVIII. The differing mode of action of emicizumab results in overestimation of its activity using existing one stage FVIII assay (OSA) and chromogenic FVIII assay (CSA). The use of CSA with bovine reagents is unaffected by emicizumab presence and is useful for estimation of baseline FVIII. A diluted one-stage assay employing emicizumab calibrator (r² diagnostics) has been validated for use in our laboratory to provide a more accurate estimation of circulating emicizumab levels in treated patients.

Aims: To assess data obtained from patients undergoing emicizumab treatment using a modified one-stage assay employing commercial emicizumab calibrator with APTT results.

Methods: The one-stage emicizumab assay, bovine chromogenic FVIII assay and APTT (ratio and seconds) were measured. The APTT results were compared with emicizumab levels in patients where baseline FVIII were found to be <1 IU/dL

Results: The APTT ratio was found to have been reduced below 1 in all patients treated with measurable emicizumab. A broad correlation was observed between emicizumab levels and APTT.

Conclusions: The most efficient method of monitoring emicizumab therapy combines the use of bovine based CSA to measure endogenous FVIII and a modified dilute OSA calibrated using commercial emicizumab reagent. However this combination of assays may not be available to all laboratories and many may rely on the APTT as a general monitor of therapy. Whilst not ideal, an APTT ratio <1 may give a broad indication of the presence of therapeutic levels of emicizumab.

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ISTH Congress Abstracts - https://abstracts.isth.org/abstract/laboratory-monitoring-of-emicizumab-in-treated-patients-how-does-aptt-compare/