Role of chromogenic assays in haemophilia A and B diagnosis

The laboratory diagnosis and monitoring of factors VIII and IX have been primarily by one-stage clotting assay (OSA) for many years. Chromogenic assays (CSA) have been available only in specialist laboratories and not for routine use. Significant differences, of more than 1.5-fold in results between the 2 assay methods, have been described in Europe and Australia in approximately one-third of patients with mild haemophilia A. In certain discrepant groups with restricted F8 gene mutations, the OSA results are more than 1.5-fold higher than CSA and risk a missed or misleading diagnostic result. More recently, an assay discrepancy in haemophilia B has been reported. With the introduction of extended half-life (EHL) FVIII and FIX products, it is likely most coagulation laboratories will need to evaluate at least one CSA and gain experience with this technique. The validation of CSA involves a careful appraisal of calibration curve linearity, limit of detection, precision, reference range, quality control material, sample analysis, method comparison and cost. This review will discuss the current status of FVIII and FIX CSA for the diagnosis of haemophilia A and B and describe approaches to implement CSA into the laboratory repertoire.