How do factor V and VIII levels affect the Chromogenix Coatest® APC Resistance tests?

A study by Colucci et al. (Thromb. Haemost. 1994, 72:987 – 988) using the classical APTT method (no FV-deficient plasma pre-dilution) showed that changes in FV levels between 12.5-100% did not modify the response to APC. Other experience has shown that the Coatest® APC Resistance V test may provide ratio values approximately 0.3 units below the median level, which can be fairly close to the cut-off value, when FV levels are 0-40%. This may be explained by the fact that although there is often no abnormal bleeding tendency in heterzygotes with factor V deficiency, prolonged PT and APTT times are observed (ref: Sartori et al. Familial association of hypoplasminogenemia and heterozygous factor V deficiency. Clin Appl Thromb Hemost. 1999; 5(4): 277-281; Salooja N et al. Severe factor V deficiency and neonatal intracranial haemorrhage: a case report. Haemophilia. 2000; 6(1): 44-46. One might also see a slightly decreased ratio in patients with severe liver disease.

According to Chromogenix, FVIII samples above 1.8 IU/ml may result in a reduction of the APC ratio of approximately 0.2 units, although the actual correlation between FVIII activity and APC ratio appears to be weak. When sampling, therefore, the patient should be at rest in order to decrease the FVIII level due to stress.