Reference Range. Partial thromboplastin time (PTT) and activated partial thromboplastin time (aPTT) are used to test for the same functions; however, in aPTT, an activator is added that speeds up the clotting time and results in a narrower reference range.he aPTT is considered a more sensitive version of the PTT and is used to monitor the patient’s response to heparin therapy. The reference range of the aPTT is 30-40 seconds. The reference range of the PTT is 60-70 seconds.