Is factor VIII (8) level the same as factor VIII (8) activity?

Factor VIII Level and Factor VIII Activity Are the Same Measurement

Yes, factor VIII level and factor VIII activity refer to the same laboratory measurement in hemophilia care. The terms are used interchangeably in clinical practice to describe the functional activity of factor VIII in plasma 1.

Understanding Factor VIII Measurement

• Factor VIII activity is the standard measurement used to classify the severity of hemophilia A, with levels reported as a percentage of normal or in international units per deciliter (IU/dL) 1
• The severity of hemophilia A is classified based on plasma levels of factor VIII activity: severe if <1%, moderate if between 1% and 5%, and mild if >5% and <40% of normal 1
• This classification correlates well with clinical bleeding profiles in most cases 1

Laboratory Testing Methods

• Factor VIII activity is typically measured using one of two main assay types:

  • One-stage clotting assay (OSCA) - the traditional and most common method based on activated partial thromboplastin time (aPTT) 1
  • Chromogenic assay (CHR) - an alternative method that may provide more accurate measurements for certain modified factor concentrates 1

• The biological potency of factor VIII is determined by an in vitro assay that employs a factor VIII concentrate standard referenced to a WHO international standard 2

Clinical Applications

• Factor VIII activity/level measurements are essential for:

  • Diagnosing and classifying hemophilia A 1
  • Monitoring response to factor replacement therapy 1
  • Detecting inhibitors (antibodies that neutralize factor VIII) 1
  • Guiding dosing decisions for factor concentrates 1

• One international unit of factor VIII activity is defined as the amount found in 1 mL of fresh pooled human plasma 2

Important Considerations

• When measuring factor VIII levels after administration of modified clotting factor concentrates (especially extended half-life products), the choice of assay method is critical as some products may give misleading results with certain reagents 1

• For monitoring treatment response, the same laboratory method should be used consistently to avoid variability in results 1

• In patients with suspected inhibitors, factor VIII activity measurements should be performed along with mixing studies and Bethesda assays to quantify the inhibitor titer 1

Potential Pitfalls

• In some laboratory reports, you may see both "factor VIII level" and "factor VIII activity" listed, but they represent the same measurement 1

• Lupus anticoagulants can cause artefactual lowering of factor levels due to inhibition of phospholipid in the assay, potentially leading to misdiagnosis 1

• Different laboratory reagents and methods may yield slightly different factor VIII activity results, especially with newer factor VIII products 1