Factor VIII (factor eight), also known as antihemophilic factor A, is one of many proteins that act in cascade to promote blood clotting. Factor VIII is produced by the liver and is transported in the blood by the von Willebrand Factor (VWF). For individuals with a hereditary or acquired factor VIII (antihemophilic factor A) (or von Willebrand Factor) deficiency, blood takes longer to clot and, as a result, bleeding is prolonged. Results are expressed in units per millilitre of blood (U/mL), a value of 1 U/mL representing 100% of the activity found in normal individuals. Below normal levels are seen in type O individuals and can be increased with exercise.
Normal levels of factor VIII indicate that an abnormality of this factor is probably not involved in the clotting disorder. Lowered activity can be inherited (hemophilia A in men, von Willebrand factor deficiency). In the elderly, an acquired factor VIII deficiency can be caused by the emergence of autoantibodies. An increase in factor VIII levels can be of genetic origin or can be caused by inflammation or damage of the liver. A level higher than 1.5 U/mL on multiple occasions may be associated with abnormal clot formation.