The prothrombin time (PT)

The test is performed by adding thromboplastin to the patient’s platelet-poor plasma, warming, and then adding calcium. The time to clot formation is recorded in seconds and the PT may be expressed as the ratio of the patient’s time to a normal control time. The thromboplastin used should have been calibrated to allow this result to be converted to the international normalised ratio (INR) – the ratio which would have been obtained if the international reference preparation for thromboplastin had been used in the test (see p. 80). The PT is essentially a measure of the efficiency of the extrinsic clotting system (factor VII) in addition to the functioning of factors V and X, prothrombin and fibrinogen.

Activated partial thromboplastin time (APTT)

This test is sometimes referred to as the partial thromboplastin time with kaolin (PTTK) or the kaolin cephalin clotting time (KCCT). Patient platelet-poor plasma is combined with contact factors (kaolin, phospholipid) and calcium and the time to clot formation recorded in seconds. The test measures the overall efficiency of the intrinsic pathway (i.e. factors VIII, IX, XI, XII) as well as the function of factors V, X, prothrombin and fibrinogen.

Quantitation of plasma fibrinogen

In most laboratories this has replaced the thrombin time as a first-line test. Several accurate methods are available for the quantitative assay of plasma fibrinogen. Fibrinogen is an acute phase reactant (see below) and is frequently elevated in sick patients. Causes of low levels include disseminated intravascular coagulation (DIC) and severe liver disease.

Common clinical causes of abnormal first-line coagulation tests are shown in Table 10.1. Second-line tests may be needed for more precise diagnosis. In mixing experiments (or correction tests) patient plasma is mixed with normal or factor-deficient plasma prior to repeating first-line tests. If a particular coagulation factor is thought to be lacking, a quantitative assay can then be performed. A circulating inhibitor of coagulation is suggested by failure of the coagulation abnormality to be corrected by the addition of normal plasma. Many routine tests are now automated. Most coagulation instruments rely on measurement of changes in optical density to detect clot formation.