The Activated partial thromboplastin time (aPTT) test tells you how many seconds (s) it takes your blood to form a clot after body tissue(s) or blood vessel walls were injured. Normally, when one of your blood vessels is damaged, proteins in your blood called clotting factors come together in a certain order to form blood clots and quickly stop bleeding. The aPTT test can be used to look at how well those clotting factors are working. It’s often used with other tests that monitor clotting factors.

Difference between the PTT and aPTT tests:

They both measure the same thing, however, in aPTT, an activator is added that speeds up the clotting time and results in a narrower reference range. The aPTT is considered a more sensitive version of the PTT and is used to monitor the patient’s response to heparin therapy.

Some reason to take this test:

- To check for a bleeding disorder like hemophilia or von Willebrand disease.

- If you get heparin therapy.

- Check that your blood clots normally before you go in for surgery

- Look for a problem with your immune system (some immune system conditions make clots more likely to form — in women, that can also lead to miscarriages)

- See how well your liver’s working, since it makes the clotting factors

Symptoms of bleeding disorders:

- Bleeding or bruising easily

- Blood clots that form when they shouldn’t

- Blood in your poop or urine

- Gums that bleed easily

- Heavy menstrual periods in women

- Nosebleeds

- Swelling or pain in your joints

Normal Ranges:

Test results are measured in seconds of time. Your results will show how long your blood took to clot, and will often compare them with results from a normal sample tested at the same time. What’s normal varies with different labs, so check with your doctor to help you understand what your numbers mean.

- A typical aPTT value is 30 to 40 seconds. If you get the test because you’re taking heparin, you’d want your aPTT results to be more like 60 to 80 seconds.

The APTT is frequently performed as part of a series of screening tests that comprise the PT, APTT and often the thrombin time and an estimation of the fibrinogen concentration.

Interfering factors:

It’s important to rule out interfering factors of an abnormal aPTT before undertaking a more detailed investigation:

- unfractionated heparin in a central venous or arterial catheter can prolong aPTT.

- A high hematocrit value will give falsely prolonged aPTT. This is due to the increased concentration of citrate relative to the small volume of plasma.

References:

- Clinical and Laboratory Standards Institute (CLSI). One-stage PT and APTT test; Approved Guideline Second Edition. H47-A2, 2008

- Greaves M, Preston FE: Approach to the bleeding patient. In Hemostasis and Thrombosis: Basic Principles and Clinical Practice. Fourth edition. Edited by RW Colman, J Hirsh, VJ Marder, et al. Philadelphia, JB Lippincott Co, 2001, pp 1197-1234