Coagulation laboratory tests

PT / INR and aPTT

Last author update: 31 January 2022
Last staff update: 31 January 2022

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PubMed Search: PT/INR AND aPTT coagulation "last 5 years"[dp]

Tori Seasor, M.D.
Karen A. Moser, M.D.
Page views in 2022: 1,876
Page views in 2023 to date: 2,038
Cite this page: Seasor T, Moser KA. PT / INR and aPTT. website. Accessed June 7th, 2023.
Definition / general
  • Prothrombin time (PT) and activated partial thromboplastin time (aPTT) are common initial tests in the evaluation of patients with suspected bleeding disorders
  • PT evaluates extrinsic and common pathways
  • aPTT measures intrinsic and common pathways
  • International normalized ratio (INR) is a calculation allowing for the standardization and comparison of PT clotting time results between laboratories
Essential features
  • PT and aPTT assays measure time to fibrin clot formation
  • PT and aPTT results are reported in seconds
  • INR is calculated from PT clotting time values and allows for standardization to monitor warfarin anticoagulant therapy
  • Coagulation screening tests
  • Clotting times
ICD coding
  • ICD-10:
    • D68.9 - coagulation defect, unspecified
    • R79.1 - abnormal coagulation profile
    • Z79.01 - long term (current) use of anticoagulants
    • Z86.711 - personal history of pulmonary embolism
    • Z86.718 - personal history of other venous thrombosis and embolism
  • Clinical & Laboratory Standards Institute (CLSI) H47-A2 includes general guidance for the performance of PT and aPTT (CLSI: H47-A2 - One-Stage Prothrombin Time (PT) Test and Activated Partial Thromboplastin Time (APTT) Test, 2nd Edition, 2008)
  • PT and aPTT are clot based coagulation tests measuring time to formation of fibrin clot (Lab Med 2017;48:295)
    • Coagulation analyzers use 1 of 2 strategies for clot detection
      • Electromechanical
      • Optical
  • Methods (Lab Med 2017;48:295, Semin Thromb Hemost 2014;40:195, Res Pract Thromb Haemost 2020;4:761):
    • Whole blood sample for testing: platelet poor plasma in 3.2% sodium citrate (collection tube with light blue top)
      • Centrifuged at 1,500 x g (3,000 RPM) for 15 minutes to separate out plasma that contains < 10,000/uL of platelets
    • PT and aPTT results are reported in seconds
    • PT: thromboplastin (tissue factor combined with phospholipid) and calcium chloride are added to the sample to initiate clotting
      • PT reagents also contain heparin neutralizers (up to 2 U/mL) (Mayo Clin Proc 2007;82:864)
      • PT measures effects of factors in extrinsic and common pathways
    • INR is a calculated value to allow for comparison of PT results from different laboratories or reagent lots (Semin Thromb Hemost 2014;40:195)

        INR = [ PTpatient ]

        • PTpatient is the measured PT of the patient in seconds
        • PTnormal is the geometric mean PT (mean normal prothrombin time, MNPT) determined from a group of at least 20 normal plasmas from healthy individuals including males and females
          • Geometric mean = antilog{[log(X1) + log(X2) + log(X3) + … + log(Xn)]/n}
        • International sensitivity index (ISI) is an experimentally derived value that is typically determined by a manufacturer for specific instrument / reagent combinations (CLSI: H47-A2 - One-Stage Prothrombin Time (PT) Test and Activated Partial Thromboplastin Time (APTT) Test, 2nd Edition, 2008)
          • International sensitivity index determination
            • WHO reference method
            • International sensitivity index = slope of the orthogonal regression line derived from comparison on logarithmic axes of PT results for local versus reference method on fresh plasma samples (20 normal plasmas and 60 plasmas from stably anticoagulated patients on vitamin K antagonists)
            • Scope of study required not practical for clinical laboratories
          • International sensitivity index reflects the sensitivity of a PT reagent to deficiencies of vitamin K dependent factors
          • Lower International sensitivity index values correlate with more vitamin K antagonist sensitive reagents
    • aPTT: a contact factor activator (e.g., kaolin, silica) combined with phospholipid and calcium chloride are added to the sample to initiate clotting
      • aPTT measures effects of factors in intrinsic and common pathways
  • Reagents (Lab Med 2017;48:295, Clin Lab Med 2009;29:253, Mayo Clin Proc 2007;82:864):
    • Reagents used for PT and aPTT testing vary between labs and show different factor sensitivities
      • PT reagents differ in phospholipid concentration and source as well as source of tissue factor
      • aPTT reagents differ in contact activator, phospholipid type and phospholipid concentration
  • Considerations when validating new reagent lots (Lab Med 2017;48:295, Clin Lab Med 2009;29:253)
    • PT
      • Must determine mean normal prothrombin time and input this into instrument or laboratory information system (LIS) for calculation of INR when new lot put into use
      • Must input new mean normal prothrombin time into instrument or LIS for calculation of INR when new lot put into use
    • APTT
      • Must determine heparin therapeutic range if aPTT will be used to monitor unfractionated heparin (UFH)
        • Heparin therapeutic range determined by measuring aPTT and UFH anti-Xa activity on a set of at least 20 plasmas from patients treated with UFH (Semin Thromb Haemost 2017;43:253)
          • Draw line of best fit
          • aPTT heparin therapeutic range represents aPTT values in seconds corresponding to UFH anti-Xa activity between 0.3 and 0.7 IU/mL
    • Optional studies
      • Determination of factor sensitivity of PT and aPTT reagents is considered a best practice but is not a required study under Clinical Laboratory Improvement Amendments (CLIA)
        • Some reagent manufacturers provide lot level information about factor sensitivity in PT and aPTT package inserts
      • Saline dilution curves help describe level of combined factor deficiencies, at which a clotting time prolongs
        • Prepared by measuring clotting time of serial dilutions of normal plasma with saline
  • Preanalytic variables (Mayo Clin Proc 2007;82:864, Lab Med 2017;48:295)
    • Variables that may affect either PT or aPTT
      • Underfilled collection tube
        • Not collecting blood to fill line
        • May occur if phlebotomist fails to account for dead space volume in a butterfly collection device
      • High hematocrit
      • Heparin contamination from a line draw
        • Tends to affect aPTT more than PT due to heparin neutralizers present in PT reagents
        • May affect PT at very high levels of heparin
      • Wrong anticoagulant in collection tube (e.g., ethylenediaminetetraacetic acid [EDTA], heparin)
    • Samples must be processed (centrifuged, plasma removed from cells and testing performed or plasma frozen for later testing) within a specified time for valid results (Semin Thromb Hemost 2019;45:443)
      • PT: 24 hours
      • aPTT (not for heparin monitoring): 4 hours
      • aPTT (for heparin therapeutic monitoring): 1 hour
Clinical features
  • PT testing: can be used clinically for evaluation of bleeding, assessment of liver disease, screening evaluation of factors II, V, VII, X and fibrinogen (Mayo Clin Proc 2007;82:864, Lab Med 2017;48:295, Res Pract Thromb Haemost 2020;4:761)
    • See Abnormal PT / aPTT values for differential diagnosis
    • Therapeutic monitoring
      • INR used to monitor warfarin therapy
      • PT / INR not used to monitor heparin or direct oral anticoagulant therapy (DOAC)
    • INR is a component of the model for end stage liver disease (MELD) score that assesses severity of end stage liver disease (MDCalc: MELD Score [Accessed 4 May 2021])
    • PT / INR may also be measured on point of care (POC) instruments
      • POC PT / INR are only approved for monitoring of vitamin K antagonists
      • May be used in limited clinical settings
        • Anticoagulation clinics
        • Home use for carefully selected patients
      • Comparison between POC PT / INR and central laboratory values may be challenging and varies by instrument and reagent
      • Possible sources of error to consider specifically with POC PT / INR performed outside of a clinical laboratory
        • Inadequate training for users
        • Inattention to quality control procedures
        • Inappropriate patient selection
        • Failure to participate in External Quality Assessment (EQA) program
        • Failure to refer unexpected or questionable results for central laboratory PT / INR
  • aPTT testing: can be used clinically for monitoring heparin and certain direct thrombin inhibitor therapy, screening evaluation of factors II, V, VIII, IX, XI, XII, fibrinogen, prekallikrein and HMWK, screening for lupus anticoagulant (lupus sensitive aPTT reagents) (Mayo Clin Proc 2007;82:864, Lab Med 2017;48:295, Res Pract Thromb Haemost 2020;4:761)
    • See Abnormal PT / aPTT values for differential diagnosis
    • Therapeutic monitoring
      • Unfractionated heparin (if aPTT has heparin therapeutic range determined by laboratory)
      • Argatroban, an anticoagulant (direct thrombin inhibitor) for treating patients with thrombosis and heparin induced thrombocytopenia
      • aPTT not used to monitor low molecular weight heparin or dabigatran therapy
  • Reference intervals are dependent upon the instrument, anticoagulant, tube type, reagent and reagent lot
  • Determination of a laboratory specific reference interval that is applicable to the current reagent lot and testing methods in use is crucial (Clin Lab Med 2009;29:253, Lab Med 2017;48:295)
    • PT results are reported in seconds with common values of 10 - 13 seconds (exact interval determined by individual laboratory)
    • aPTT results are reported in seconds with common values 25 - 40 seconds (exact interval determined by individual laboratory)
    • INR
      • INR improves comparison of prolonged PT values between laboratories for the purpose of monitoring warfarin therapy
      • INR therapeutic range for warfarin = 2.0 to 3.0 for most indications (Blood Adv 2020;4:4693)
  • Critical values
    • Determined by individual laboratories but may be similar to the following recommended values from the International Council for Standardization in Haematology (ICSH) (Semin Thromb Hemost 2020;46:398)
      • PT / INR: INR 4.0 - 6.0
      • aPTT: should be locally determined
Board review style question #1
The laboratory supervisor tells you that a new lot of reagent is to be started next week and the international sensitivity index (ISI) needs to be updated. If this is done incorrectly, which laboratory result(s) will be affected?

  1. aPTT
  2. INR
  3. PT
  4. PT and aPTT
  5. Thrombin time
Board review style answer #1

The international normalized ratio (INR) is used to standardize PT across different laboratories to monitor warfarin therapy. The international sensitivity index (ISI) value is specific to a reagent lot and analyzer. The INR is then calculated by the following equation:

INR = [ PTpatient ]

Comment Here

Reference: PT / INR and aPTT
Board review style question #2
A patient has an unexpectedly prolonged aPTT of > 150 seconds and a normal PT. The patient is not receiving any anticoagulants and has no history of bleeding or thrombosis. The laboratory investigates this unexpected value and identifies a prolonged thrombin time of > 150 seconds. What is the most likely preanalytic variable that would explain this pattern of results?

  1. EDTA anticoagulant in tube
  2. Heparin contamination
  3. High hematocrit without citrate volume correction
  4. Underfilled collection tube
Board review style answer #2
B. Heparin contamination

The marked prolongation of both the aPTT and thrombin time in this case is best explained by heparin contamination. Although all of the other preanalytic variables listed could cause falsely prolonged clotting times, they would be expected to also falsely prolong the PT (which was normal in this case) and may not show such extreme aPTT and thrombin time prolongation as can be seen with heparin.

Comment Here

Reference: PT / INR and aPTT
Board review style question #3
Your laboratory is provided a reagent lot with an international sensitivity index (ISI) of 1.31 on your instrument. The PT is performed with this reagent in 40 normal donors. With this information, the arithmetic mean is calculated to be 12.9 seconds and the geometric mean is calculated is 13.0 seconds. A patient sample PT is 18.1 seconds using this reagent lot in your laboratory. What is the international normalized ratio (INR) for this patient sample?

  1. 1.2
  2. 1.5
  3. 1.6
  4. 1.8
Board review style answer #3
B. 1.5

The calculated patient INR for this given ISI and PTnormal is 1.5, with the PTpatient (18.1 seconds) and the PTnormal being the PT of normal donors using the geometric mean (13.0 seconds) using the formula:

INR = [ PTpatient ]

Comment Here

Reference: PT / INR and aPTT
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