The haemostatic system comprises four compartments: the vasculature, platelets, coagulation factors, and the fibrinolytic system. There is presently no laboratory or near-patient test capable of reproducing the complex regulated interaction between these four compartments. The prothrombin time (PT) and activated partial thromboplastin time (APTT) only test the coagulation protein compartment of the system and results have to be carefully interpreted in the context of the clinical presentation and assay limitations. This article will give a general overview of the limitations of PT and APTT and discuss specific issues that need to be considered when the tests are requested, in the context of anticoagulant monitoring, bleeding symptoms, and routine preoperative screening. Of these indications, routine preoperative screening is the most controversial and is generally not warranted in the absence of an abnormal bleeding history.