|Add exactly 2.7 mL of blood & mix by inversion.
Deliver to laboratory within 60 minutes
When the collection is by vacuum tube,the coagulation tube should be collected second to avoid artefactual shortening of coagulation assays due to the trauma of venesection. When syringes are used, have ready two plastic syringes. A butterfly needle may be used. Withdraw 5 mL blood into the first syringe and discard the syringe (or use it for other tests). With the second syringe, withdraw the required amount of blood for the coagulation tests. When blood is collected from an arterial line it is necessary to discard the first 20mL of blood to clear any heparin from the line before collecting the blood for coagulation tests.
This test is insensitive to therapeutic doses of Low Molecular Weight Heparin (see ANTI-Xa ASSAY and ANTICOAGULANT THERAPY CONTROL).
APTT may be artefactually shortened if the venesection is traumatic or artefactually prolonged if the tube is 'under-filled' or the patient has a raised haemoglobin/haematocrit.
Specimens cannot be analysed or re-analysed greater than 2 hours after collection.