Activated Partial Thromboplastin Time (APTT)

Activated partial thromboplastin time (aPTT) is a laboratory test that measures the time it takes plasma to clot via the intrinsic and common coagulation pathways. It’s performed on citrated plasma with an activator and phospholipid after calcium is added. It helps detect factor deficiencies, monitor unfractionated heparin therapy, and evaluate unexplained bleeding or clotting; prolongation suggests factor defects, anticoagulants, liver disease, or lupus anticoagulant.

A high activated partial thromboplastin time (aPTT) means the intrinsic and common coagulation pathways are working more slowly. It can reflect anticoagulant therapy (e.g., heparin), clotting‑factor deficiencies (e.g., hemophilia), liver disease or consumptive disorders like DIC. Sometimes antiphospholipid antibodies prolong aPTT despite thrombosis risk. Clinically it raises bleeding concern and prompts medication review, mixing studies and specific factor testing or hematology referral.

A low PTT means blood clots faster than expected. It can indicate a hypercoagulable state — from high factor VIII or fibrinogen, pregnancy, inflammation, oral contraceptives, or malignancy — and raises risk of thrombosis (DVT, PE). Pre‑analytical issues (short draw) can also shorten PTT. Clinicians typically repeat testing and investigate underlying causes and thrombotic risk.

PTT (partial thromboplastin time) is an older assay of the intrinsic and common coagulation pathways. aPTT (activated PTT) adds a contact activator and standardized reagents, shortening run time and improving reproducibility; it’s the routine test used to assess intrinsic pathway factor deficiencies and to monitor unfractionated heparin. In practice “aPTT” is the standard clinical test.

Activated partial thromboplastin time (APTT) is typically about 25–40 seconds in most laboratories. Exact reference ranges vary by lab and assay—many report roughly 25–35 or 30–40 seconds. Values prolonged above the lab’s upper limit suggest clotting factor deficiencies, inhibitors, or anticoagulation (for example, heparin). Shortened times are uncommon. Always interpret results against the local laboratory reference range.

APTT can be prolonged by anticoagulants (heparin, direct thrombin/Xa inhibitors), intrinsic‑pathway factor deficiencies (VIII, IX, XI, XII), factor inhibitors (acquired hemophilia, lupus anticoagulant), severe liver disease, disseminated intravascular coagulation, advanced vitamin K deficiency, massive transfusion/dilutional coagulopathy, hypothermia, or sampling errors. A shortened APTT may reflect high clotting factor levels or inadequate anticoagulant in the sample.