ACTIVATED PARTIAL THROMBOPLASTIN TIME (APTT) - plasma

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Specimen: 4.5 mL blood added to 0.5 mL citrate. The sample must not be collected into heparinised syringes or tubes, or from heparinised lines. See Table 1 .

Method: Recalcification of platelet-poor plasma at 37°C in the presence of an activator and platelet substitute. Measure time to clot.

Reference Interval: Commonly 25-35 seconds, but varies with reagents and method - consult pathologist. The therapeutic interval for continuous infusion heparin is generally 1.5-2.5 x the baseline or control APTT, but also varies with the reagents used and should be checked with the laboratory.

Application: Inappropriate as a routine preoperative screening test, due to its limited sensitivity and specificity. Used as an initial test when the personal and/or family history suggest a coagulation factor deficiency or when the history suggests a coagulation factor inhibitor or a lupus inhibitor. A baseline APTT prior to heparin therapy may detect a lupus inhibitor. The APTT is used to monitor full dose continuous infusion IV heparin therapy but should not be used to monitor “prophylactic” subcutaneous heparin or low molecular weight heparin. See Bleeding , Anticoagulant monitoring .

Interpretation: A normal APTT does not exclude mild, but clinically significant, coagulation factor deficiency (eg as in mild haemophilia, vWd) as many reagents give a prolonged APTT only at coagulation factor levels £30%. An isolated prolongation of the APTT (PT normal) suggests deficiency of factor VIII, IX, XI or XII. Prolongation of both the APTT and PT suggests factor X, V, II or I (fibrinogen) deficiency, all of which are rare. The APTT is normal in factor VII deficiency (PT prolonged) and factor XIII deficiency. See Figure 1 . A prolonged APTT which is not corrected by the in vitro addition of normal plasma suggests a coagulation factor (VIII or IX) inhibitor or a lupus inhibitor. Artefactual prolongation of the APTT may be due to the presence of heparin in the sample, difficult or slow collection, addition of an incorrect volume of blood to the tube, delay in mixing blood with the citrate anticoagulant, suboptimal specimen storage or a prolonged interval between collection and testing. Consult pathologist.

Reference: Suchman AL and Griner PF. Ann Intern Med 1986; 104: 810-816. Basu D et al. N Engl J Med 1972; 287: 324-327.

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