Our patient has factor XII deficiency. Factor XII is needed in the activation of the clotting cascade when measuring the APTT in the laboratory (via contact factor). However it is not required in life to activate clotting. Hence people do not bleed. However it can lead to anxiety, especially when routine coagulation screens are abnormal. Different laboratories use different reagents when measuring the APTT so some may be more sensitive than others in picking up factor XII deficiency. As it does not result in bleeding the operation can proceed without any further intervention and the patient should receive prophylactic LMWH as per protocol.
Factor XII deficiency should be suspected in someone with an isolated prolonged APTT which corrects on mixing studies (or does not correct with a lupus insensitive reagent if this is what your laboratory will do first) and where there is no bleeding history personally or in the family. The APTT will correct to normal when mixed with normal plasma as the missing factor(s) present in the normal plasma will allow clot to form.
The patient should be informed of the diagnosis since the APTT will be prolonged and this may cause unnecessary delays to essential procedures.
This case highlights some important points:
- 1) History is key to diagnosing bleeding disorders – is the patient well or unwell? Is there liver disease? On anticoagulants?
- 2) Coagulation screens should not be performed indiscriminately (our patient was having a tendon repair and without any bleeding history did not require a pre op coagulation screen)
- 3) Repeating the specimen is the first priority and then proceeding to sequential testing to establish the cause of the abnormality (mixing studies here would be a good first line step)
- 4) Looking at old results may help (has further investigations previously been done or if the APTT has been normal in the past may point towards an acquired disorder)
- BCSH guidelines on coagulation tests pre op: http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2141.2007.06968.x/epdf