Factor VIII activity can be measured in different ways. Two of the assays used to assess the Factor VIII activity are the one-stage clotting assay and the chromogenic assay.
The one stage clotting assay measures the extent a plasma sample corrects the coagulation time of a Factor VIII deficient plasma in an APTT based assay.
The first step of the chromogenic assay, “patient plasma (containing an unknown amount of functional F VIII) is added to a reaction mixture consisting of thrombin or prothrombin, F IXa, FX, calcium and phospholipid. This nearly immediately produces FVIIIa, which works in concert with FIXa to activate FX. When the reaction is stopped, FXa production is assumed to be proportional to the amount of functional F VIII present in the sample based on the a standard curve.”
First, the chromogenic FVIII activity does not show interference from lupus anticoagulants (LA), making it the ideal choice for accurately measuring FVIII activity in patients with LA. Additionally, a subset of mild hemophilia A patients demonstrates higher FVIII activity in one-stage clot vs. two-stage or chromogenic FVIII activity assays where results may be significantly lower. The bleeding phenotype in these patients is more closely related to the lower FVIII activity.2
Since discrepancies in the FVIII assay are common, it is recommended, “All hemophilia treatment center coagulation laboratories should use both one-stage and chromogenic FVIII activity assays, especially when screening for possible hemophilia A.”
 Moser, Karen A. and Funk (Adcock), Dorothy M. “Chromogenic factor VIII activity assay”. Am J Hematol. 2014 Jul;89(7):781-4. doi: 10.1002/ajh.23723. Epub 2014 Apr 15.
 Potgieter, Joachim J, Damgaard, Michael and Hillarp, Andreas. One-stage vs. chromogenic assays in haemophilia A. European Journal of Hematology, Volume 94, Issue s77, February 2015, p38-44.