aPTT
Also written as: activated partial thromboplastin time, PTT, APTT
Times a second clotting pathway, used to monitor heparin and to screen clotting factors.
What it measures
How many seconds a plasma sample takes to clot when a different arm of the clotting system than the PT test is triggered in the lab. It reflects the combined activity of several clotting factors working in sequence.
Why a doctor orders it
Ordered to monitor the blood thinner heparin, before some operations, and to investigate unexplained bleeding, bruising, or clotting. It is often run alongside the PT so the two clotting pathways can be compared.
What is generally associated with a higher result
Broadly associated with slower-than-average clotting. Recognized causes include heparin, shortages of specific clotting factors such as those seen in hemophilia, and antibodies called lupus anticoagulant, which despite the name are linked to clotting rather than bleeding and often occur without lupus.
What is generally associated with a lower result
Broadly associated with a sample that clotted quickly, which can simply reflect how the specimen was collected. Low values are generally not pursued.
Everyday things that move this number
Not every shift in a value reflects disease. These are ordinary factors known to affect this particular measurement:
- A slow or difficult blood draw, which can activate clotting in the tube
- An underfilled collection tube
- Contamination from an IV line that has been flushed with heparin
- Delay before the lab processed the sample
Questions to take to your appointment
- Was the sample drawn through an IV line that could have contained heparin?
- Would a mixing study - a follow-up test that blends my plasma with normal plasma - help show whether a clotting factor or an antibody is involved?
- How does this result fit alongside my PT and INR?
Build a printable questionnaire with your own results →
Where to read more
MedlinePlus, from the US National Library of Medicine, publishes patient-facing pages on individual lab tests and is a reliable next stop: medlineplus.gov/lab-tests .
Support this site
This site is free, has no paywall, and takes no money from hospitals, insurers, or billing companies. If it helped, a coffee keeps it going.
Buy us a coffee