By Haley Masterson
While PTT and PT will both be affected by disorders of the common pathway of coagulation cascade, the PTT is used as a lab test marker for the function of the intrinsic branch of the coagulation cascade, and the PT for the extrinsic.
Confused yet? Don’t be. Try using this heartbreaking visual mnemonic:
PTT (sometimes also called Activated Partial Thromboplastin Time) measures the intrinsic pathway, because two Ts in the picture above are in a relationship. This test measures all of the coagulation factors except for Factor VII and Thrombin (Factor XIII), and when prolonged can be especially notable as indicating hemophilia or (sometimes) von Willebrand’s Disease.
The PT, on the other hand, measures the extrinsic pathway, because the two Ts are no longer together. They are exes. (Poor “P” is merely a bystander to this romantic tragedy). The PT is used to test Factor 7 (the extrinsic pathway factor).
To remember why Factor VII (7) is associated with PT and not PTT, you have two options:
1. The Scientific Reason: The “P” in Partial Thromboplastin Time stands for “Partial”. Why? Because unlike the PT, the PTT actually doesn’t include Tissue Factor – which is critical in activating 7 to 7a. So by definition, the PTT can’t measure the activity of Factor 7 – there’s no TF to activate it. Therefore Factor 7 must be measured by the PT test.
2. The Extension of the Mnemonic: Factor 7 is in the extrinsic pathway because the infamous “7 year itch” is what caused the two Ts to become exes.
1. Kumar, Vinay, and Stanley L. Robbins. Robbins Basic Pathology. 8th ed. Philadelphia, PA: Saunders/Elsevier, 2007.
2. Le, Tao, and Vikas Bhushan. First Aid for the USMLE Step 1 2010. New York: McGraw-Hill Medical, 2010.