With a good understanding of the physiology we can now hopefully apply this to understand the methods used to testing coagulation in clinical practice.
This test is utilised to assess the extrinsic pathway as well as the final common pathway of coagulation. As you can probably guess, to test the extrinsic pathway, the prothrombin time involves adding tissue factor (a.k.a. thromboplastin) to the patient's blood sample. As the blood sample was citrated at collection, calcium is also added to allow the coagulation process to occur.
As you can see from the pathway, this will provide a test of factors VII, X, V, Prothrombin (II), and fibrinogen. In general, it is factor VII deficiency that has the greatest role in prolonging the PT compared with these other clotting factors. The time until clotting is normally between 10 and 14 seconds. It is important to note that this time is strongly affected by the reagent used, as well as the laboratory equipment, and thus can show a large degree of variability between labs. To overcome this problem, the PT is usually expressed as the International Normalised Ratio (INR). This involves correction of the PT time value obtained using a correction formula based on the specific reagent and equipment used by that lab (as below). A normal INR is 1.0.
The International Sensitivity Index (ISI) is a normalising value provided for the laboratories reagent compared with a WHO reference preparation. The mean PT is calculated from a minimum of 20 healthy patients tested at that laboratory.
Activated Partial Thromboplastin Time (APTT)
The APTT is a test of the intrinsic pathway, as well as the final common pathway. Phospholipid and calcium are added to the collected citrated blood, alongside and activator substance e.g. kaolin). This triggers the intrinsic pathway but not the extrinsic pathway (as tissue factor isn't present). This therefore tests factors XII, XI, IX, VIII and the common pathway factors X, II, V and fibrinogen.
The normal range is 30-40 seconds. Partial thromboplastin time refers to the same test but without the addition of the activator substance. This is generally slower and the APTT is felt to be more sensitive.
This provides an assessment of the final common pathway, and particularly of fibrinogen. An excess of thrombin is added to the blood sample and the time to clotting is measured. This will detect problems with fibrinogen or inhibitors of thrombin e.g. heparin.
The normal range is 14-16 seconds.
Others to be aware of...
A Full Blood Count is a vital part of every coagulation test given the vital function of platelets in the whole process. Although it won't provide information on platelet function it can help identify thromocytopenia. The Bleeding Time is a historical test that simply measured the time taken for bleeding to cease from a small cut (usually on the forearm with a blood pressure cuff inflated). The normal time range was 3-8 minutes. It has been superseded by more accurate testing.