What does PTT measure?

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The main concept being tested is how long it takes for a fibrin clot to form after initiating the intrinsic pathway. In this test, plasma is exposed to an activator that starts the contact factors and calcium is added; the time from activation to clot formation is measured. This clotting time reflects the functioning of the intrinsic pathway (factors XII, XI, IX, VIII) and the common pathway (factors X, V, II, I). Prolongation can indicate deficiencies in those factors or the presence of inhibitors such as heparin or lupus anticoagulant, making it useful for monitoring unfractionated heparin therapy and for evaluating intrinsic pathway integrity. It does not measure platelet aggregation or directly quantify fibrinogen levels—the former is a platelet function question, and the latter is assessed by other tests such as thrombin time or Clauss.