Important practical pitfalls in testing
Uncritical interpretation of laboratory results leads to misdiagnosis and thrombophilia testing exemplifies this. If erroneous and overdiagnosis are to be avoided, the following points must be recognised:

Normal ranges for antithrombin and proteins C and S are wide and patients with deficiency may have levels that are only slightly below normal. Repeat testing is often required for diagnostic confidence. For example, laboratory quality assurance data have shown that for protein S in particular the rate of laboratory error in diagnosis can be as high as 20%.41

Pregnancy induces a state of resistance to the anticoagulant effect of activated protein C, which mimics the presence of factor V Leiden.

Pregnancy and OCP use lead to a fall in plasma protein S concentration.

Antithrombin concentration is reduced in acute thrombosis, by heparin treatment and in pre?eclampsia.

Proteins C and S are vitamin K dependent and their concentrations are reduced by warfarin treatment.

Even if the above potential pitfalls are avoided, there is no indication to test for heritable thrombophilia at presentation with acute VTE, as management is not affected by the results.