Coagulation Factor Assays(Download  Coagulation Factor Assays.pdf) These notes are produced for HAPS Hub partners to streamline the ordering of appropriate tests in patients where a coagulation factor deficiency is a possibility. The clinical Haematologists of HAPS are available to discuss any questions arising from these guidelines or patients in whom appropriate tests are not obvious. Screening Tests For Bleeding Disorders 1. The activated partial thromboplastin time (APTT) is the clotting test that screens the intrinsic pathway of the coagulation cascade. The most common cause of an isolated prolongation of APTT is a lupus anticoagulant. Factor deficiencies in this pathway that prolong the APTT and are associated with bleeding are Factors VIII, IX, XI, and rarely Passovoy factor. If the APTT is prolonged a 50:50 correction with normal plasma should automatically have been performed by the lab (see 4). 2. The prothrombin time ( PT) is the clotting test that screens the extrinsic pathway. An isolated prolongation of PT is due to Factor VII deficiency. If the PT is prolonged a 50:50 correction should automatically have been performed by the lab (see 4). 3. Prolongation of both APTT and PT is most commonly caused by liver disease, Vitamin K deficiency or warfarin therapy, and rarely by Factors V and X. If the APTT and PT are prolonged a 50:50 correction on both should automatically have been performed by the lab (see 4). 4. 50:50 corrections are performed by the laboratory on all samples with abnormal results. A mixture of normal pooled plasma and the test plasma is made in a 50:50 ratio. The abnormal test is then repeated using the mixture. Any factor deficiency should show full correction of the test result. Inhibitors show no or only partial correction. 5. Platelet count for thrombocytopenia. There is no value in bleeding times as a screen for bleeding disorders. 6. These tests do not screen for von Willebrand’s disease (see specific disorders below). 7. All screening tests should be repeated at least once to confirm any abnormality. The most sensitive screen for inherited bleeding disorders is the personal and family history. Clinical Scenarios There are several different scenarios where factor assays are appropriate.
Clinical Scenario |
Factor Assays |
Prolonged APTT with 50:50 correction and normal PT. |
Tests to request are Factors VIII, IX, and XI assays, but history may point to a specific factor. |
Prolonged PT with 50:50 correction and normal APTT |
Test to request is Factor VII assay |
Prolonged APTT and PT with correction of both. |
Tests to request are Factors V, and X assays, once liver disease, Vitamin K deficiency and warfarin therapy are excluded. |
Prolonged APTT which does NOT correct with 50:50 mix. |
This is almost always due to an inhibitor, most commonly a lupus anticoagulant. Factor assays are of no value in this scenario. Testing for an inhibitor is appropriate. |
Personal bleeding history. |
Screening with APTT, PT, and platelet count is appropriate. Then proceed according to abnormality detected. Normal screening tests do NOT exclude von Willebrand’s disease (see tests for specific disorders below) or platelet functional disorders. |
Family history of bleeding disorder. |
Identify the bleeding disorder and order the appropriate test for the deficiency. | |