With the rapid onset of game-changing injectable emicizumab [HEMLIBRA] therapy for hemophilia, coagulation laboratories face a new challenge. On the occasions when HEMLIBRA-treated hemophiliacs need factor VIII concentrate therapy, or if there is a desire to test for residual factor VIII, what means does the laboratory possess? Likewise, the clinician may need a means to check for a factor VIII inhibitor, what is available? Please look to the right for our April, 2020 Quick Question, “How does your facility measure factor VIII during HEMLIBRA therapy?” Select your answer and please provide us with detailed information on your facility’s approach, using the Comments section below.
[Many thanks to Dr; Ali Sadeghi-Khomami for his comprehensive April 4, 2020 comment below.]
I think the following information could assist laboratories in choosing the right assay for testing a plasma sample obtained from a person treated with Hemlibra with or without FVIII.
Emicizumab (Hemlibra) is a bi-specific humanized antibody that recognizes human coagulation factors (FIXa and FX). Thus, it mimics the role of FVIIIa in the human coagulation system. When a patient’s sample suspected to contain both Hemlibra and FVIII, routine one stage clot-based assays (OSC) cannot be used for accurate quantitative measurements due to mechanistic interferences. In such cases, if there is a desire to measure residual FVIII activity alone, labs should use any routine plasma calibrator with a two-stage chromogenic substrate FVIII assay (CSA), which are not sensitive to Hemlibra. For example, a CSA kit based on solely bovine factors or a mixture of bovine FX with human FIXa is effective. If there is a need to measure a Hemlibra level when mixed with an unknown FVIII, then at first labs should apply a heat deactivation step to the plasma sample (56°C, 30 minutes in a water-bath, centrifuge the vials for 5 min at 2700 x g at room temperature, use supernatant) then quantify Hemlibra using a product-specific calibrator and a solely human-based CSA kit. There is also a modified FVIII OSC assay available that could estimate Hemlibra level. This research use only product relies on the assumption that there is no significant residual FVIII in samples, thus super-diluted samples won’t be impacted significantly by residual FVIII (not true for all cases). Measurement of FVIII inhibitor antibodies (BU) in plasma samples containing Hemlibra could also be done after the heat deactivation step described above followed by Bethesda assay using a non-sensitive Hemlibra CSA as the endpoint FVIII activity measurement.
As a representative example, the following specialty commercial products could be used by laboratories when they are dealing with Hemlibra containing plasma samples based on each applicable scenario described above:
Non-sensitive Hemlibra FVIII CSA:
Solely bovine-based: Siemens FVIII Chromogenic Assay, Chromogenix Coamatic or Coatest SP4 FVIII
Hybrid: PrecisionBioLogic CRYOcheck Chromogenic Factor VIII
FVIII calibrator and controls:
PrecisionBioLogic CRYOcheck Normal Reference Plasma, Abnormal 1 & 2 Reference Controls
Sensitive Hemlibra FVIII CSA:
Solely human-based: Biophen FVIII:C
Hemlibra specific calibrator and controls:
r2 Diagnostics Emicizumab calibrator and controls
FVIII inhibitor kit:
PrecisionBioLogic CRYOcheck Factor VIII Inhibitor Kit