The coagulation cascade shown on the previous slide can be overwhelming. For the laboratory professional, it is more important to group which factors are in each section of the cascade rather than memorize the whole cascade. Knowing a table like the one below can help you quickly group each factor into intrinsic, extrinsic, and common.
Knowing which factors are in the extrinsic, intrinsic, and common pathway is helpful, as the coagulation screening tests (e.g., PT and aPTT) assess different pathways. Thus, an elevation in the PT and/or aPTT can narrow down which coagulation factors may be deficient.
- The PT assay is specific for the EXTRINSIC pathway - factors III and VII. Deficiencies in factor VII will result in a prolonged PT result. Note: Factor III, also known as thromboplastin, is also part of the extrinsic pathway. However, it is added to start the PT reaction, so the patient's thromboplastin level is not measured with this test.
- The aPTT assay is specific for the INTRINSIC pathway - factors VIII, IX, XI, and XII. Deficiencies in these factors will result in a prolonged aPTT result.
- Deficiencies of COMMON pathway - factors X, V, II, and I can result in a prolonged PT AND aPTT.
- Liver disease causes a decrease in the production of all proteins. Since most coagulation factors are proteins, you will see a prolonged PT AND aPTT in severe liver disease.
