APTT

The range depends on the reagent the laboratory uses (some use 22 to 35 or 27 to 40 seconds); use the range your lab printed.

The APTT measures how many seconds it takes plasma to clot after a different trigger from the prothrombin time. It tests the other main arm of the clotting system, including factors VIII, IX, XI and XII, and it is the test used to monitor unfractionated heparin. A longer time means the blood is slower to clot by this route.

Doctors request it before operations, when someone bruises or bleeds easily, when looking into inherited clotting factor shortages such as haemophilia and von Willebrand disease, when monitoring heparin given by drip, and as part of a lupus anticoagulant screen.

• Heparin, particularly unfractionated heparin by drip
• Inherited shortages of clotting factors, including haemophilia and von Willebrand disease
• Lupus anticoagulant, an antibody that lengthens the test in the tube
• Liver disease
• Some direct oral anticoagulant medicines
• A severe illness that uses up clotting factors
• Low vitamin K

• A high factor VIII level, which rises with inflammation, pregnancy and oestrogen
• Recent surgery, injury or inflammation, slightly
• A sample that has begun to clot in the tube

• An under-filled or slowly filled citrate tube reads long
• A sample taken from a line that has been flushed with heparin reads long
• Different reagents give different times, so results are not comparable between laboratories
• A shortage of factor XII lengthens it markedly in the tube yet causes no bleeding tendency

No fasting needed. Taken in a blue-top citrate tube filled to the line, ideally not from a heparin-flushed line.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.