Chromogenic Factor VIII, Activity

Last Literature Review: December 2019 Last Update:
  • Measures hydrolysis of a p-nitroanilide (pNA) substrate
    • The rate of release of pNA is proportional to the factor VIII activity in the sample
  • Can quantitate factor activity as low as 1% of normal

Chromogenic factor VIII activity is used for the diagnosis of nonsevere hemophilia A (in conjunction with the one-stage clot-based factor VIII activity). Nonsevere hemophilia A may require both one-stage clot-based factor VIII activity and chromogenic factor VIII activity for diagnosis and severity classification due to differences in how the underlying mutations affect factor VIII activity in the tests. 

This assay may also be used in the measurement of factor VIII activity in the presence of interfering drugs or lupus anticoagulants that result in underestimation by clot-based methods.

For more information about the hemophilia A and the recommended laboratory testing strategy, refer to the ARUP Consult Hemophilia - Factor VIII or IX Deficiency topic.

Disease Overview

Incidence

Hemophilia A in 1/4,000-5,000 live male births worldwide; rare in females , 

Inheritance

X-linked recessive, factor VIII deficiency can also be acquired due to autoantibodies. , 

Visit the Hemophilia - Factor VIII or IX Deficiency Consult topic for additional information about factor VIII deficiency.

Diagnostic Issues

Hemophilia A may be classified as mild, moderate or severe, based on factor activity. 

Expected Factor VIII Activity in Hemophilia A
Disease ClassificationExpected Factor Activity
Mild6-40%
Moderate1-5%
Severe<1%

Monitoring Issues

Modified extended half-life factor VIII replacement products may lead to under- or overestimation of factor VIII activity in clot-based factor VIII assays using certain aPTT reagents. , ,  For more information about the effects of specific factor VIII replacement products on factor VIII activity, refer to the Effects of Extended Half-Life Factor VIII Replacement Products on Factor VIII Activity table.

Additionally, factor VIII activity cannot be accurately measured using a one-stage clot-based factor VIII activity assay in the presence of emicizumab.  Measurement of factor VIII activity or factor VIII inhibitors in the presence of emicizumab requires a chromogenic assay using bovine reagents (either a chromogenic factor VIII activity or a chromogenic factor VIII Bethesda assay). The ARUP chromogenic factor VIII activity assay uses bovine reagents and is not affected by emicizumab.

Test Interpretation

Results

  • Age-specific reference intervals are provided for each result on the patient chart.
  • Decreased factor VIII activity is expected in patients with hemophilia A (refer to the Expected Factor VIII Activity in Hemophilia A table for disease classification) and is associated with increased risk of bleeding.

Limitations

  • Decreased chromogenic factor VIII activity results may also be caused by:
    • von Willebrand disease
    • Specimen collection and storage issues:
      • Uncontrolled freeze-thaw cycles
      • Prolonged ambient storage
      • Activated or clotted specimens
    • Anticoagulant medications (assay interference)
      • Heparin (>2 U/mL)
      • Direct thrombin inhibitors
      • Direct Xa inhibitors
  • Factor VIII activity may be elevated above usual baseline (normal or high result could mask underlying deficiency) in patients with acute phase responses.
  • Normal factor VIII activity does not exclude female hemophilia carrier status.

References