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The Difference Between APTT & PTT

There are numerous laboratory tests that measure how long it takes for the blood to clot. There are two common serum analyses primarily used to monitor heparin therapy. These are the activated partial thromboplastin time (aPTT) and partial thromboplastin time (PTT). Moreover, both of these are utilized for screening coagulation deficiencies and detection of coagulation inhibitors. The ability to understand and differentiate aPTT from PTT is essential for accurate interpretation of the results.

  1. Laboratory Values

    • The normal values vary from different institutions and laboratories for aPTT and PTT. aPTT ranges from 30 to 40 seconds. While under anticoagulant treatment, the therapeutic level is 2.0 to 2.5 times the normal value. On the other hand, PTT is around 60 to 70 seconds. It is observed that PTT is longer compared to aPTT. Examples of clinical problems that may increase the aPTT are hemophilia, cirrhosis and leukemia. A reduced aPTT may take place with extensive cancer.

    Older and Newer

    • The predecessor of aPTT was the PTT. Richard Ravel, the author of Clinical Laboratory Medicine: Clinical Application of Data, states that PTT was first introduced and used as a detector of intrinsic abnormalities. It was also applied in observing the effects of heparin therapy. As it was later discovered, PTT became insufficient in detecting minor deficiencies and had decreased heparin sensitivity. Thus, aPTT emerged to cover what PTT was lacking.

    Added Factor

    • By adding certain chemicals or "contact activators" such as kaolin to the reagent of PTT, it proved to activate factor XII rapidly and consistently. The reagent is a negatively charged activator, such as silica and a phospholipid. Hence, this finding removed another clotting factor and added a reagent which in turn hastened the clotting time.

    Increased Sensitivity

    • PTT is used to screen for coagulation disorders. It aims to reveal any defects in the intrinsic coagulation system and common pathways. Furthermore, it seeks to assess any deficit in the clotting factors II, V, VIII, IX, XI, and XII. In contrast, aPTT is more sensitive in terms of its effect to heparin. In accordance to Fischbach and Dunning, aPTT "is a more sensitive version of PT." Consequently, aPTT is used to monitor heparin management. It is regularly used to find if there are any factor VIII and IX deficiencies.

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