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The Activated Partial Thromboplastin Time (APTT) test — also called PTT — measures the time taken for blood to clot through the...
Also known as: APTT Test · aPTT Test · Activated Partial Thromboplastin Time · PTT Blood Test · Heparin Monitoring Test
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*Inclusive of home collection charges
The Activated Partial Thromboplastin Time (APTT) test — also called PTT — measures the time taken for blood to clot through the intrinsic and common coagulation pathways. It is the primary test for detecting deficiencies in intrinsic pathway clotting factors (Factors VIII, IX, XI, XII, and common pathway factors V, X, and II/prothrombin), and is the standard laboratory assay for monitoring unfractionated heparin (UFH) anticoagulation therapy. It is essential for diagnosing haemophilia A (Factor VIII deficiency) and haemophilia B (Factor IX deficiency). No fasting required.
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What Does the Partial Thromboplastin Time (PTT) / APTT Test Measure?
| Level / Result | Possible Interpretation |
|---|---|
| 25–35 seconds (Normal) | Normal intrinsic and common pathway coagulation function |
| Mildly prolonged (35–50 seconds) | Borderline — mild factor deficiency, lupus anticoagulant, or early DIC; repeat and investigate |
| Moderately prolonged (50–80 seconds) | Significant coagulation factor deficiency — haemophilia likely if isolated; DIC if with other abnormalities |
| Markedly prolonged (> 80 seconds) | Severe factor deficiency, high-dose heparin, or severe DIC — urgent evaluation and specialist review |
| APTT prolonged + PT normal | Isolated intrinsic pathway defect — haemophilia A or B, Factor XI or XII deficiency, heparin therapy |
| APTT prolonged + PT prolonged | Common pathway defect or combined deficiency — severe liver disease, DIC, Vitamin K deficiency, warfarin overdose |
| APTT prolonged + No correction on mixing study | Presence of an inhibitor — lupus anticoagulant or Factor VIII inhibitor — not a factor deficiency |
Important Note: APTT results must be interpreted alongside the PT/INR, clinical history, and a mixing study (patient plasma + normal plasma) to distinguish factor deficiency from inhibitor. APTT is highly sensitive to heparin — always note current anticoagulation therapy before interpreting. Sample collection technique is critical — underfilled citrate tubes give falsely prolonged results. All results must be reviewed by a physician or haematologist.
The Mixing Study — Distinguishing Deficiency from Inhibitor: When APTT is prolonged, a mixing study adds equal parts of the patient's plasma and normal pooled plasma. If the APTT corrects to normal — the deficiency is corrected by normal plasma's clotting factors, confirming a factor deficiency. If APTT does not correct — an inhibitor is present (a substance blocking coagulation). The most clinically important inhibitor is the lupus anticoagulant (associated with antiphospholipid syndrome — paradoxically causes thrombosis despite prolonging APTT). Factor VIII inhibitors (acquired haemophilia) are another important cause. This distinction completely changes clinical management.
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