Partial Thromboplastin Time (PTT) Test What is a PTT (partial thromboplastin time) test? A partial thromboplastin time (PTT) test measures the time it takes for a blood clot to form. Normally, when you get a cut or injury that causes bleeding, proteins in your blood called coagulation factors work together to form a blood clot. The clot stops you from losing too much blood. You have several coagulation factors in your blood. If any factors are missing or defective, it can take longer than normal for blood to clot. In some cases, this causes heavy, uncontrolled bleeding. A PTT test checks the function of specific coagulation factors. These include factors known as factor VIII, factor IX, factor X1, and factor XII. What is it used for? A PTT test is used to: Check the function of specific coagulation factors. If any of these factors are missing or defective, it can mean you have a bl...