What is being tested?The partial thromboplastin time (PTT) test measures the functionality of the intrinsic and common pathways of the
coagulation cascade. When a person starts bleeding, the body uses the coagulation cascade to produce blood clots to seal off injuries to blood vessels, to prevent further blood loss and to give the damaged areas time to heal. The cascade consists of a group of
coagulation factors produced in the liver. These proteins are activated sequentially along either the extrinsic (tissue-related) or intrinsic (blood vessel-related) pathways. The branches of the pathway then come together into the common pathway and complete their task with the formation of a stable blood clot.
Each component of the coagulation cascade must be functioning properly and be present in sufficient quantity for normal blood clot formation. If there is an inherited or acquired deficiency in one or more of the factors, or if the factors are functioning abnormally, then stable clot formation will be inhibited and excessive bleeding and/or clotting may occur.
The PTT test measures the length of time that it takes for clotting to occur in a test tube when reagents (chemicals) are added to
plasma. When the sample takes longer than normal to clot, the PTT is said to be “prolonged.”
