Blood Coagulation Tests and Interpretations
Bleeding time assays are used to evaluate the vascular and platelet responses that are associated with hemostasis. The bleeding time is a frequent assay performed on preoperative patients to ensure there is an adequate response to vessel injury prior to surgery. As indicated above, the rapid responses to vascular injury (occurring within seconds) are vessel constriction and platelet adhesion to the vessel wall. The Ivy method for determining the bleeding time involves the use of a blood pressure cuff (sphygmomanometer) which is placed on the forearm and inflated to 40mmHg. A superficial incision is then made on the forearm and the time it takes for bleeding to stop is recorded. With the Ivy method bleeding should stop within 1–9 minutes. Any bleeding time greater than 15 minutes would be indicative of a defect in the initial responses of vessels and platelets to vascular injury. A less invasive bleeding time assay involves the use of a lancet or special needle and a 3–4mm deep prick is made on the fingertip or earlobe. This bleeding time assay is referred to as the Duke method and in this assay bleeding should cease within 1–3 minutes. The bleeding time is affected (prolonged) by any defect in platelet function, by vascular disorders, and in von Willebrand disease but is not affected by other coagulation factors. Disorders that are commonly associated with an increased bleeding time include thrombocytopenia, disseminated intravascular coagulation (DIC), Bernard-Soulier syndrome and Glanzmann thrombasthenia. Abnormal bleeding times are also found in patients with Cushing syndrome, severe liver disease, leukemia, and bone marrow failure.
Defects associated with factors of the pathways of blood coagulation can also be assessed with specific assays. The prothrombin time (PT) is an assay designed to screen for defects in fibrinogen, prothrombin, and factors V, VII, and X and thus measures activities of the extrinsic pathway of coagulation. When any of these factors is deficient then the PT is prolonged. A normal PT is 11.0–12.5 seconds. A PT greater than 20 seconds is indicative of coagulation deficit. The PT is measured using plasma after the blood cells are removed. A blood sample is collected in a tube containing citrate or EDTA to chelate any calcium and thus inhibit coagulation and then the cells are removed by centrifugation. After the cells are removed excess calcium is added to the plasma to initiate coagulation. The most common measure of PT is to divide the time of coagulation of a patients blood by that of a known standard and this value is referred to as the international normalized ratio (INR). Normal INR values range from 0.8–1.2. PT is used to determine the correct dosage of the warfarin class of anti-coagulation drugs (e.g. Coumadin), for the presence of liver disease or damage, and to evaluate vitamin K status.
The partial thromboplastin time (PTT) is used to assay for defects in the intrinsic pathway of coagulation. The PTT assay has been modified by the addition of activators that shorten the normal clotting time and this form of the assay is referred to as the activated partial thromboplastin time (aPTT). The PTT is normally prescribed in patients with unexplained bleeding or clotting. The assay will evaluate the function of fibrinogen, prothrombin, and factors V, VIII, IX, X, XI, and XII. A defect in any of these factors will result in a prolonged PTT (or aPTT). A normal PTT is 60–70 seconds, whereas for the aPTT the normal range is 30–40 seconds. The PTT is a standard assay used to assess the efficacy of heparin anticoagulant therapy. Prolonged PTTs are associated with acquired or congenital bleeding disorders associated with coagulation factor deficiency, vitamin K deficiency, liver disease, DIC,von Willebrand disease, leukemia, hemophilia, and during heparin administration.
No comments:
Post a Comment