Abstract
The advent of modified factor VIII (FVIII) and factor IX (FIX) molecules with extended
half-lives (EHLs) compared with native FVIII and FIX represents a major advance in
the field of hemophilia care, with the potential to reduce the frequency of prophylactic
injections and/or to increase the trough level prior to subsequent injections. Monitoring
treatment through laboratory assays will be an important part of ensuring patient
safety, including any tailoring of prophylaxis. Several approaches have been used
to extend half-lives, including PEGylation, and fusion to albumin or immunoglobulin.
Some of these modifications affect factor assays as routinely performed in hemophilia
centers; so, laboratories will need to use FVIII and FIX assays which have been shown
to be suitable on a product-by-product basis. For some products, there are marked
differences between results obtained using one-stage or chromogenic assays and results
obtained using different reagents in the one-stage assay. The laboratory should use
an assay in which the recovery of the product closely aligns with the assay used by
the pharmaceutical company to assign potency to the product, so that the units reported
by the laboratory agree with those used to demonstrate efficacy of the product during
clinical trials. Reported assay differences in relation to several of the EHL FVIII
and FIX molecules will be reviewed in this article.
Keywords
Factor VIII assay - FIX assay - extended half-life FVIII - extended half-life FIX