Dispelling myths about coagulation abnormalities in internal medicine

Abstract

The clotting screen is an 'integral' part of the routine blood tests in most medical wards. It is likely that only with the increasing requests for prothrombin time and activated partial thromboplastin time are abnormal results noted. Interpretation of these results requires good understanding of the coagulation system and problems with the laboratory analysis. Due to variable understanding of this complex system, many misconceptions have arisen in relation to the clinical effects expected from abnormal clotting screens. Some of these are discussed with considerations of appropriate management in those situations.

Keywords: Coagulation; plasma; prothrombin time; thromboplastin time.

Fig 1.

Investigation for prolonged prothrombin time (PT) or activated partial thromboplastin time (APTT) or both. In the case of an isolated prolonged PT, or both PT and APTT, intravenous vitamin K 10 mg may help in ‘diagnosing’ vitamin K deficiency and correct the abnormal tests. A mixing study involves mixing the plasma of an individual with known normal levels of coagulation factors with that of the patient. If the tests normalise it suggests deficiency of a clotting factor. If it does not, it suggests, commonly, lupus anticoagulant or, rarely, an inhibitor to coagulation factors. The latter is time and temperature dependent, ie an initial correction of APTT to normal may be noted but on continued incubation at 37°C, it becomes prolonged again. *One of the most common reasons for an abnormal APTT with a normal PT is inadequately filling the blood specimen tube. Repeating the test after ensuring that the specimen tube is appropriately filled is advisable before proceeding to further work-up. ^†Lupus anticoagulants can prolong the PT in some instances through anti-prothrombin activity. In such cases both the PT and the APTT are prolonged. PT = prothrombin time; APTT = activated partial thromboplastin time.