The PT INR APTT test is a blood test used to measure how long it takes for the blood to clot. The test is performed by collecting a blood sample from a vein in the arm and then placing it in a tube with a substance that triggers clotting. The time it takes for the blood to clot is measured and compared to a standard range to determine if there is a bleeding or clotting disorder. This test is commonly used to monitor patients taking blood thinning medications.