This question was posted on the Diagnostic Equity website on March 5, 2026: “My PT is normal at 12 seconds, but my PTT (aPTT, APTT) is elevated at 72 seconds. I had excessive bleeding after a dental procedure. My hematologist said a mixing study was done and it was corrected, so what does a mixing study tell us, and how does ‘correction’ narrow down the diagnosis?”
Kathryn Golab, DCLS, responded,
Prothrombin time (PT) and Activated Partial Thromboplastin Time (aPTT, APTT, or PTT) are two tests that are used to help determine how well you can form a clot. The clotting process is a complicated series of steps that occur in three phases: platelet aggregation, formation of the fibrin clot, and breakdown of the fibrin clot. PT and PTT both measure the second phase, formation of the fibrin clot.
There are over a dozen coagulation factors that are activated in a specific order. Two different pathways occur as a part of the formation of the fibrin clot: the extrinsic and the intrinsic pathway, which come together to form a common pathway that leads to fibrin formation. PT measures the function of the coagulation factors in the extrinsic pathway, and the PTT measures the function of the intrinsic pathway. The fact that the PT was normal shows that there are no issues with your extrinsic pathway or common pathway.
A prolonged PTT indicates that there is an issue with the intrinsic pathway. There are several possibilities, and a mixing study is important in the investigation of the issue. Your specimen is mixed in equal parts with what is called normal pooled plasma, which contains the plasma of blood donors who don’t have bleeding problems. The laboratory then performs the PTT on the mixed specimen to see if the result “corrects”, or gives a value within the normal range 9reference interval). If the PTT is corrected, there is a deficiency in one of the coagulation factors within the intrinsic pathway. The most common deficiencies are in factor 8 and factor 9. Deficiencies in either one of these factors are known as hemophilia.
Hemophilia can be both hereditary (you inherit the gene for it from one or both of your parents) or acquired. In severe cases, hereditary hemophilia first appears in childhood. Acquired hemophilia usually occurs due to the development of an antibody against factor 8 or 9, which reduces the amount of the factor and its ability to function correctly. If the result did not correct, that would have been indicative of an inhibitor, which means that there is something in your blood that is preventing your clotting factors from functioning correctly and forming that fibrin clot.
Your providers will likely do some more blood tests to determine the levels of your clotting factors and to test for the presence of inhibitors. Here are some additional questions that you can ask your hematologist for clarity:
- My PTT was corrected as part of the mixing study; does this mean I have a clotting factor deficiency?
- Do you know what type of clotting factor deficiency I have?
- Are there additional tests that need to be done to see what is causing the issue?
- Based on this information, do I have a specific bleeding disorder?
- Do I need to be concerned about excessive bleeding againcafter other procedures?
- Is there any treatment that I need? What and how often?
Thanks to Dr. Golab and to Health Equity Emissary Team (HEET) member Tamara Lobban-Jones for permission to post this information.
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