A high Partial Thromboplastin Time (PTT) result means your blood is taking longer than expected to form a clot. This test measures your body’s ability to stop bleeding, involving a sequence of protein interactions known as the coagulation cascade. When the PTT value is elevated, it indicates a defect in this process, suggesting a potential risk of excessive bleeding or an underlying disorder. The result is not a final diagnosis, but rather points toward a problem with the clotting factors in your blood.
What the PTT Test Measures
The PTT test evaluates the function of the “intrinsic” and “common” pathways of the coagulation cascade. These pathways involve chemical reactions that lead to the formation of a stable fibrin clot. The test measures the time, in seconds, it takes for a clot to form in a blood sample after certain reagents are added.
The test checks the activity of several clotting factors, including Factors VIII, IX, XI, and XII (intrinsic pathway), and Factors X, V, II (prothrombin), and I (fibrinogen) (common pathway). The normal reference range for a PTT test is typically between 25 and 35 seconds, though this range varies slightly by laboratory. A result exceeding this upper limit is considered prolonged or high, indicating that one or more of these factors may be deficient or malfunctioning.
Common Reasons for an Elevated Result
An elevated PTT can stem from medical conditions or therapeutic interventions that affect clotting factor function or quantity. One common reason is the therapeutic use of anticoagulant medications, or “blood thinners.” Unfractionated heparin, often used in hospitals to prevent clots, works by slowing the clotting process. Monitoring PTT levels is standard practice to ensure the heparin dose is within a safe range, often purposefully prolonged to 60 to 100 seconds.
A high PTT can also signal a deficiency in one or more clotting factors. Inherited bleeding disorders, such as Hemophilia A (Factor VIII deficiency) and Hemophilia B (Factor IX deficiency), are classic examples. Both conditions prolong the PTT because these factors are necessary for the intrinsic pathway to function correctly.
Acquired conditions also cause factor deficiencies. Severe liver disease impairs the liver’s ability to produce many clotting factors, including those measured by the PTT test. A lack of Vitamin K, required for the synthesis of Factors II, VII, IX, and X, can also prolong the PTT, though this deficiency also affects the Prothrombin Time (PT) test.
Another significant cause is the presence of a circulating inhibitor, an antibody that interferes with normal clotting. The most frequent is the Lupus Anticoagulant, a type of antiphospholipid antibody. This antibody binds to phospholipids used in the laboratory test, artificially prolonging the PTT result in vitro. Paradoxically, the presence of Lupus Anticoagulant is associated with an increased risk of blood clot formation in vivo.
Next Steps After a High PTT Reading
Since a prolonged PTT is an indicator, the next step involves further diagnostic testing to pinpoint the specific cause. The initial follow-up test is the Coagulation Mixing Study, which determines if the problem is a factor deficiency or the presence of an inhibitor. This test involves mixing the patient’s plasma with normal plasma, which contains 100% of all clotting factors.
If the PTT of the mixed sample corrects (falls into the normal range), it suggests a factor deficiency because the normal plasma supplied the missing factor. If the PTT remains prolonged, it indicates a circulating inhibitor is present, as the inhibitor continues to interfere with the supplied clotting factors.
The next step is often Specific Factor Assays to measure the exact level of individual clotting factors, such as Factor VIII or IX. The management plan depends entirely on the underlying cause identified, whether it is adjusting an anticoagulant dose, treating an acquired condition, or managing an inherited disorder. Consulting with the ordering physician or a hematologist is necessary to interpret these results within the context of the patient’s medical history.