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 memorizing the whole cascade. Knowing a table, such as the one below, can help you quickly group each of the factors into intrinsic, extrinsic, and common.
Knowing which factors are in the extrinsic, intrinsic, and common pathway is helpful, as the coagulation screening tests (eg, 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 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.
- The PT assay is specific for the EXTRINSIC pathway- factors III and VII. Deficiencies in these factors will result in a prolonged PT result.
- Liver disease causes a decrease in the production of all proteins. Since most all coagulation factors are proteins, you will see a prolonged PT AND aPTT in severe liver disease.
