Prolonged PT or APTT Interpreter — Which Factor, Which Next Test
Prolonged PT or APTT Interpreter — Which Factor, Which Next Test
Enter whether the PT and the APTT are prolonged against your laboratory’s intervals, with the context that changes the answer — anticoagulants including DOACs, liver disease, bleeding or an acutely unwell patient, a known lupus anticoagulant, a doubtful sample. It names the pathway, the factors that could be low and the next test, including the prolonged APTT that never bleeds (factor XII) and the one that is an emergency (acquired haemophilia A).
Prolonged PT or APTT
PT + APTT + context → pathwayPT normal, APTT prolonged, no anticoagulant, incidental finding before surgery, no liver disease, no known lupus anticoagulant, sample not in doubt
The three patterns
| Pattern | Pathway | Factors | Common causes | Bleeds? |
|---|---|---|---|---|
| PT long, APTT normal | Extrinsic | VII | Vitamin K deficiency, early warfarin, liver disease, early DIC, factor VII deficiency | Depends on the cause |
| APTT long, PT normal | Intrinsic | VIII, IX, XI, XII, prekallikrein, HMWK | Heparin, dabigatran, lupus anticoagulant, factor XII deficiency, haemophilia, von Willebrand disease, acquired haemophilia A | Not with factor XII, prekallikrein, HMWK or lupus anticoagulant; yes with VIII, IX, XI |
| Both long | Common, or several factors | X, V, II, fibrinogen | Warfarin excess, DIC, liver disease, severe vitamin K deficiency, high-dose heparin, DOACs | Often |
Read the pattern first, then the context
The PT and the APTT each start the clotting cascade at a different point and end at the same place, so which of them is long tells you where to look. The PT tests factor VII plus the common pathway (X, V, II and fibrinogen); the APTT tests the contact factors, XI, IX and VIII plus the same common pathway. An isolated prolonged PT therefore points at factor VII, an isolated prolonged APTT at the intrinsic factors, and both together at the common pathway or at several factors falling at once.
Context then does most of the work. Anticoagulants are the commonest explanation of a long clotting time in hospital, and the DOACs are the least predictable: the 2025 ADLM guidance warns that a normal PT or APTT cannot exclude them, particularly apixaban. Before any investigation, the sample itself has to be right — an underfilled citrate tube or a haematocrit above 0.55 prolongs both tests (see the citrate volume adjustment calculator).
The two opposite errors this page is built to prevent are both in the isolated APTT. Factor XII deficiency can produce a strikingly long APTT and never causes bleeding, so it should not delay surgery or trigger plasma. Acquired haemophilia A produces the same pattern in an older or postpartum patient with new bruising and can be fatal; the incubated mixing study is how the laboratory picks it up — see the coagulation mixing study interpreter. A lupus anticoagulant sits between them: it prolongs the APTT in the tube and causes thrombosis in the patient.
For warfarin, use the INR calculator; for heparin monitoring, the APTT ratio calculator; for the acutely ill patient with both tests prolonged, the ISTH DIC score calculator.
Frequently asked questions
What causes a prolonged APTT with a normal PT?
A defect in the intrinsic pathway: heparin or dabigatran, a lupus anticoagulant, a deficiency of factor VIII, IX, XI or XII or of the contact factors, von Willebrand disease lowering factor VIII, or an inhibitor such as acquired haemophilia A. Without bleeding, lupus anticoagulant and factor XII deficiency are the commonest; with bleeding, factors VIII, IX and XI.
Does factor XII deficiency cause bleeding?
No. Factor XII, prekallikrein and high-molecular-weight kininogen deficiencies can prolong the APTT markedly but are not associated with clinical bleeding. They need no treatment and no special precautions for surgery, once a genuine bleeding factor deficiency has been excluded.
What causes a prolonged PT with a normal APTT?
Factor VII is the one factor measured by the PT and not the APTT, so an isolated prolonged PT usually means a fall in factor VII: vitamin K deficiency, early warfarin, early liver disease or DIC, or inherited factor VII deficiency. Rivaroxaban and edoxaban also prolong the PT more than the APTT.
Can a DOAC be present with a normal PT and APTT?
Yes. The ADLM 2025 guidance states that normal PT and APTT results cannot exclude a DOAC, and apixaban in particular barely affects most PT reagents. A drug-calibrated anti-Xa level measures apixaban, rivaroxaban and edoxaban; a normal thrombin time excludes dabigatran.
What does a mixing study add?
Mixing the patient’s plasma 1:1 with normal plasma corrects a factor deficiency, because the normal plasma supplies the missing factor, but not an inhibitor. Incubating the mix for two hours at 37 °C reveals time-dependent inhibitors such as factor VIII antibodies. See the coagulation mixing study interpreter.
Related calculators
References
- ARUP Consult. Prolonged Clotting Time Evaluation. ARUP Laboratories, accessed September 2026. — isolated prolonged PT: "FVII deficiency or inhibitor; in some cases, warfarin, vitamin K deficiency, liver disease, or DIC"; isolated prolonged APTT without bleeding: "FXII, prekallikrein, HMWK (not associated with clinical bleeding)"; both prolonged: DIC, severe liver disease.
- Kamal AH, Tefferi A, Pruthi RK. How to interpret and pursue an abnormal prothrombin time, activated partial thromboplastin time, and bleeding time in adults. Mayo Clin Proc. 2007;82(7):864–873. — the pathway-by-pathway approach this page follows (the full text could not be retrieved while this page was written; the specific statements are sourced to the other references).
- Bazydlo L, et al. ADLM guidance document on coagulation testing in patients using direct oral anticoagulants. J Appl Lab Med. 2025. doi:10.1093/jalm/jfaf155. — "normal PT and/or aPTT results cannot be used to definitively exclude the presence of a DOAC"; "the PT is less sensitive to apixaban than it is to rivaroxaban"; dabigatran lowers PT- "and especially aPTT-based" factor results; "a normal TT rules out the presence of dabigatran", and the TT is unaffected by factor Xa inhibitors.
- Devreese KMJ, de Groot PG, de Laat B, et al. Guidance from the Scientific and Standardization Committee for lupus anticoagulant/antiphospholipid antibodies of the International Society on Thrombosis and Haemostasis. J Thromb Haemost. 2020;18(11):2828–2839. — lupus anticoagulant detected with two tests based on different principles (dilute Russell’s viper venom time and an LA-sensitive APTT).
- Tiede A, Collins P, Knoebl P, et al. International recommendations on the diagnosis and treatment of acquired hemophilia A. Haematologica. 2020;105(7):1791–1801. — acquired haemophilia A to be considered in a patient with new bleeding and an isolated prolonged APTT.
- Clinical and Laboratory Standards Institute. Collection, Transport and Processing of Blood Specimens for Testing Plasma-Based Coagulation Assays and Molecular Hemostasis Assays. CLSI H21-A5, 2008. — citrate tubes filled to at least 90%; citrate adjusted when the haematocrit is above 55%.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.