Prolonged Clotting Time Evaluation

Clotting time tests such as prothrombin time (PT) and/or activated partial thromboplastin time (aPTT) are commonly performed in an outpatient or hospital setting to evaluate the hemostatic system for a variety of reasons.  Symptoms associated with prolonged clotting times depend on the underlying etiology.  Prolonged clotting times of unclear etiology require further evaluation to determine the cause and exclude clinically significant bleeding disorders.  Panel tests with reflex testing for the workup of unexpected prolonged clotting times can reduce the number of times blood will need to be drawn to establish a diagnosis  and can lead to more rapid diagnosis. It is important to note that anticoagulant medications may affect testing, as can a recent blood transfusion or factor replacement, and appropriate specimen handling is critical to ensure accuracy of results.

Quick Answers for Clinicians

Which laboratory tests should be used for the workup of prolonged clotting times?

Appropriate testing depends on the clinical presentation (bleeding, thrombosis, or asymptomatic presentation), underlying medical conditions and medications, and which clotting times are prolonged (prothrombin time [PT], activated partial thromboplastin time [aPTT], etc.).  Additional testing could include lupus anticoagulant evaluation, fibrinogen, d-dimer, factor assays, Bethesda assays, or von Willebrand factor testing, depending on the clinical scenario.

What factors can affect coagulation test results?

Anticoagulant medications frequently interfere with coagulation testing.  Refer to the Anticoagulants and Possible Coagulation Test Interferences table for more information. Therapies such as blood products and coagulation factor replacement can also affect results.

Indications for Testing

Individuals who have been noted to have a prolonged clotting time (such as PT and/or aPTT) should undergo further evaluation if the cause of the prolonged clotting time is uncertain. 

Laboratory Testing

Initial Testing

Obtaining a CBC with platelet count may be useful (eg, to help identify or exclude quantitative platelet abnormalities).  The following tests (available as a reflex panel in some laboratories) are appropriate for patients with prolonged PT and/or aPTT but no strong clinical or laboratory evidence of a specific hemostasis disorder:

  • PT, aPTT
  • Dilute Russell viper venom time (DRVVT)
  • Lupus anticoagulant reflexive testing
  • Fibrinogen
  • D-dimer

Observed patterns in the initial test results determine additional or reflexive testing, which may include coagulation factor assays, Bethesda assays, and/or von Willebrand factor testing.

Condition-Specific Testing

Condition-specific testing is preferred in cases of a known coagulation disorder or a strong suspicion of a particular disorder. In patients with a bleeding presentation, consider testing for factor deficiencies, such as hemophilia A or B or rare factor deficiencies. The patterns of the PT and aPTT results should guide test selection.  As shown in the table below, normal PT and aPTT results are seen with some bleeding disorders.

In patients with a thrombotic presentation, consider lupus anticoagulant testing.

In patients with an asymptomatic presentation, consider lupus anticoagulant testing, testing for factor deficiencies not associated with a bleeding presentation (eg, factor XII, prekallikrein, and high-molecular-weight kininogen [HMWK]), and testing for mild factor deficiencies. 

PT aPTT Platelet Count Potential Cause
N N FVII deficiency or inhibitor; in some cases, warfarin, vitamin K deficiency, liver disease, or DIC; in some cases, oral direct Xa inhibitors
N N Deficiency or inhibitor of intrinsic pathway, eg, FVIII, FIX, FXI  (associated with clinical bleeding), or FXII, prekallikrein, HMWK  (not associated with clinical bleeding); VWD types with low FVIII activity; heparin; in some cases, oral direct thrombin inhibitor, or lupus anticoagulant in patients with asymptomatic or thrombotic presentation
N Warfarin, vitamin K deficiency; liver disease; DIC; high heparin concentration; direct thrombin inhibitor; direct Xa inhibitor; common pathway deficiency versus inhibitor (FII, FV, FX, fibrinogen); rare lupus anticoagulants with hypoprothrombinemia; multiple factor deficiencies
DIC; severe liver disease; rare cases of heparin-induced thrombocytopenia
N N Thrombocytopenia (decreased production, increased destruction, or splenic sequestration)
N N N Vascular abnormalities; VWD; qualitative platelet disorder; mild factor deficiency; dysfibrinogenemia; FXIII deficiency; fibrinolytic disorders; in some cases, direct oral anticoagulants
DIC, disseminated intravascular coagulation; F, factor (eg, factor VII, or FVII); N, normal; VWD, von Willebrand disease

Test Interference

Coagulation tests may be inaccurate in the presence of anticoagulant drugs or other interfering substances.  Refer to the Anticoagulants and Possible Coagulation Test Interferences table for possible interferences with different coagulation tests based on the specific drug administered.

ARUP Lab Tests

Medical Experts



Karen A. Moser, MD
Assistant Professor of Clinical Pathology, University of Utah
Medical Director, Hemostasis/Thrombosis, ARUP Laboratories


Kristi J. Smock, MD
Associate Professor of Clinical Pathology, University of Utah
Medical Director, Hemostasis/Thrombosis, ARUP Laboratories


Additional Resources
Resources from the ARUP Institute for Clinical and Experimental Pathology®