Coagulation Studies

The most common coagulation studies, prothrombin time (PT) and partial thromboplastin time (PTT), are used to evaluate patients with clotting disorders or to monitor patients taking heparin or oral anticoagulants. It is helpful for the laboratory to know whether a patient is taking an anticoagulant at testing. Hospitalized patients with nonsurgical diagnoses, who do not have liver disease or a history of anticoagulant use, do not benefit from routine PT and PTT testing. These are poor screening tests for postoperative bleeding in patients without historical risk factors, physical findings, or a medication history that suggests an increased bleeding risk (Eckman et al., 2003). Preoperative PT and PTT should be reserved for patients with known or suspected coagulation disorders and those receiving anticoagulation therapy.

Prothrombin time, a simple and inexpensive test for evaluating the extrinsic coagulation pathway, is the time in seconds for citrated plasma to clot after the addition of calcium and thromboplastin. Test accuracy depends on proper collection and instrument technique. Common uses include monitoring anticoagulant therapy with warfarin, evaluating liver function (because the liver synthesizes most of the clotting factors), and screening for coagulation disorders of the extrinsic system. PT is prolonged by defects in factors I (fibrinogen), II (prothrombin), V, VII, and X. The normal range for PT is 11 to 13 seconds.

Previously, PT measurements exhibited variability across laboratories because of differences in thromboplastin sensitivity. To correct for the type of thromboplastin used, the World Health Organization (WHO) recommends using the international normalized ratio (INR) to report PT results for patients taking oral anticoagulants. Now widely accepted, the INR is calculated as follows:

INR = (Patient PT / Mean normal PT)ISI

The ISI is the international sensitivity index of the thromboplastin used at the local laboratory. Provided by the test's manufacturer, ISI reflects the responsiveness of the thrombo-plastin used in the PT test. The reference range for the INR in the non-anticoagulated patient is 0.9 to 1.1. The PT is prolonged in persons with vitamin K deficiency, including those with fat malabsorption syndromes, recent broad-spectrum antibiotic use, and premature infants. In addition, the use of warfarin, many drugs and herbs, severe liver disease, alcoholism, deficiencies of clotting factors, and circulating anticoagulants can prolong the PT. The PT is not affected by platelet disorders or platelet count. The target INR varies with specific indications. In general, an INR goal of 2.5 (range, 2.0-3.0) is generally accepted for the treatment of venous thromboembolic disease and atrial fibrillation, and 3.0 (range, 2.5-3.5) for patients at risk for arterial thromboembolism, including those with mechanical heart valves.

The activated partial thromboplastin time (aPTT, or simply PTT) is a simple, inexpensive test for evaluating the intrinsic coagulation pathway, monitoring heparin therapy, screening for hemophilia A and B, and detecting clotting inhibitors. PTT is the time in seconds for citrated plasma to clot after a contact activator is added to plasma and incubated at 37° C for 5 minutes. Thromboplastin and calcium are added and the time to clot formation is recorded, which should be within 10 seconds of the control. PTT is abnormally prolonged in most patients with coagulation disorders (~90%) and is therefore the best screening test in persons suspected of having a clotting disorder. PTT screens for all coagulation factors that lead to thrombin formation except VII and XIII. These factors include factors I, II, V, VIII (antihemophiliac), IX (Christmas), X, and XII (Hageman). PTT is useful to evaluate patients with a known, suspected, or active bleeding disorder; consumptive coagulopathy (e.g., disseminated intravascular coagulation); disorder of fibrin clot formation; or fibrinogen deficiency. In addition, PTT is prolonged with deficiency of the Fletcher (prekallikrein) and Fitzgerald factors, warfarin or heparin therapy, lupus anticoagulant, and vitamin K deficiency. PTT is significantly shortened by hemolysis, is affected by high or low hematocrit, but is not affected by platelet dysfunction or count. A prolonged PT or PTT can be caused by either a factor inhibitor or a deficiency of a clotting factor. To differentiate the two, a mixing study can be performed. When the abnormality is corrected after mixing with normal blood, a factor deficiency is likely. Failure to correct after mixing suggests the presence of a factor inhibitor.

When monitoring heparin therapy, the most widely used target for anticoagulation is a PTT 1.5 to 2.5 times the upper limit of normal. Now, however, because of the great variation in thromboplastins used in different PTT assays, PTT results vary widely among laboratories. Therapeutic heparin levels, as measured by antifactor Xa units, are approximately 0.3 to 0.7 antifactor Xa IU/mL. With plasma concentrations of heparin at 0.3IU/mL, investigators have found that mean PTT values ranged from 48 to 108 seconds, depending on the laboratory methods used. The American College of Chest Physicians recommends against the use of a fixed PTT therapeutic range for the treatment of venous thrombosis; instead, they recommend that each laboratory determine the PTT range that corresponds to a therapeutic heparin level: 0.3 to 0.7 IU/mL by factor Xa dilution. Anti-factor Xa levels may also be used to monitor appropriate anticoagulation doses in patients with obesity or renal failure, because these groups are more likely to be over-anticoagulated using weight-based heparin dosing (Hirsch et al., 2008).

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