What It Is
Activated partial thromboplastin time, commonly abbreviated as aPTT, is a blood test that measures how long it takes for blood to clot, similar in general purpose to PT/INR but assessing a different part of the body's clotting process.
What It Checks
The aPTT test measures the time it takes for a blood sample to form a clot under specific laboratory conditions, reflecting the activity of a different set of clotting factors than those measured by PT/INR. Together, PT/INR and aPTT provide a more complete picture of the overall clotting process, since each test is more sensitive to different clotting factors.
Why It's Used
aPTT is commonly used to assess the blood's general clotting ability, to investigate unexplained bleeding or clotting problems, and to monitor certain blood-thinning medications that specifically affect the part of the clotting process this test measures.
How It's Done
The test requires a blood sample, typically drawn from a vein in the arm, similar to most routine blood tests. The sample is sent to a laboratory, where specific reagents are added under controlled conditions to measure how long clot formation takes.
Background
Along with prothrombin time, aPTT was developed through blood clotting research conducted over the mid-20th century, as scientists identified the many individual clotting factors involved in the overall clotting process and developed specific laboratory tests sensitive to different parts of that process. Together, these two tests became standard tools for evaluating clotting function, and aPTT remains a routinely used test today, particularly in monitoring certain anticoagulant medications.