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Prothrombin Time with INR

Test code(s) 8847

Question 1. My patient, who is not taking an anticoagulant, has an elevated PT-INR. But on a repeat test, the result was within range (0.9-1.1). Can you explain the discrepancy in test results?

Sampling and blood collection problems (pretest variables) usually account for the discrepancy. The most common pretest variables are:

  • Underfilled, uncapped, or expired collection tubes1
  • Hemolysis (most common to syringe or butterfly needle collections)
  • Clotted or partially clotted specimen (failure to mix specimen after collection)

A comprehensive review of pre-analytic variables (ie, collection, processing, storage) and their impact on plasma-based coagulation assays can be purchased from the CLSI (Document H21-A5 [clsi.org]).

Question 2. Other than warfarin (eg, Coumadin®), what else might cause a prolonged PT-INR result?

A prolonged prothrombin time with INR result can be caused by a medical condition, medications, and other factors.

Medical conditions:

  • Vitamin K deficiency, which may be secondary to malabsorption or antibiotic therapy; administration of vitamin K will lead to a significantly shorter PT or normalization within 12-24 hours
  • Liver disease (due to diminished production of clotting factors)
  • Hematocrit >55% (common in newborns and patients with polycythemia vera)
  • Disseminated intravascular coagulopathy (DIC)
  • Dysfibrinogenemia (production of abnormal fibrinogen)/abnormal fibrinolysis
  • Clotting factor deficiencies (factors II, V, VII, X, fibrinogen)
  • Young age and hematocrit ≤55% (Note: Full term neonates less than 3 months of age will have slightly increased PT/INR due to decreased coagulation factors)

Drugs:

  • Direct thrombin inhibitors (ie, dabigatran, argatroban)
  • Factor Xa inhibitors (rivaroxaban, apixaban, edoxaban, fondaparinux)
  • Excessive unfractionated heparin (ie, >2.0 IU/mL) (however, low molecular weight heparin [LMWH] will cause minimal or no elevation)

Other:

  • Gross lipemia
  • Markedly elevated bilirubin
  • Lupus anticoagulants. Most PT-INR reagents contain excess phospholipid that neutralizes lupus anticoagulants. However, strong lupus anticoagulants can cause a mild prolongation or accentuate the prolongation of the PT when patients are on warfarin. In these situations, a chromogenic factor X assay (test code 10663) can be used rather than, or in addition to, the PT-INR to monitor warfarin.

Question 3. Which test should I request to follow up on a persistently prolonged PT-INR result?

Consider testing for vitamin K-dependent clotting factors (II, VII, X) and a nonvitamin K-dependent clotting factor (V).  When combined with clinical history, these tests can help differentiate a coagulopathy due to vitamin K deficiency, hepatic insufficiency, or a single factor deficiency.

Depending on the clinical context, a fibrinogen or DIC evaluation (D-dimer, antithrombin III activity, platelet count, fibrinogen, aPTT, and PT) may be helpful.

Question 4. What are the critical values at Quest Diagnostics?

Quest Diagnostics does not use the term “critical values” because this implies an understanding of the medical context of each patient. We use the term “priority values.” We have 2 different levels which trigger different notification protocols. All priority 1 results are promptly called 24 hours daily.

We established the priority values after considering the following:

  • Some therapeutic indications target a higher intensity PT-INR (INR of 3.0 to 4.0).
  • The likelihood of bleeding has been reported to rise steeply as the PT-INR increases above 5.0.2  However, data from a large registry of warfarin-treated patients suggest that the short-term risk for major bleeding is low (0.96% after 1 month of therapy) for someone with a single INR value between 5.0 and 9.0.3

Table. Priority Value Definitions

Question 5. How is the INR calculated?

The INR is calculated using the following formula:

The ISI of the PT reagent used at Quest Diagnostics is ~1.0.

Question 6. Is the PT-INR valid for patients with liver disease?

Yes. The PT-INR is generally prolonged in patients with stable chronic liver disease, because clotting factor production is diminished. But if the liver disease is unstable, the concentration of clotting factors will be variable, and the PT-INR would be an unreliable measure.

References

  1. Adcock DM, Kressin DC, et al. Minimum specimen volume requirements for routine coagulation testing: dependence on citrate concentration. Am J Clin Pathol. 1998;109:595-599.
  2. Ageno W, Gallus AS, Wittkowsky A, et al. Oral anticoagulant therapy: antithrombotic therapy and prevention of thrombosis, 9th ed: American College of Chest Physicians evidence-based clinical practice guidelines.Chest. 2012;141(suppl 2):e44S-e88S.
  3. Garcia DA, Regan S, Crowther M, et al. The risk of hemorrhage among patients with warfarin-associated coagulopathy. J Am Coll Cardiol. 2006;47:804-808.
This FAQ is provided for informational purposes only and is not intended as medical advice. A physician’s test selection and interpretation, diagnosis, and patient management decisions should be based on his/her education, clinical expertise, and assessment of the patient.
Document FAQS.104 Version: 1
Version 1 effective 01/11/2017 to present
Version 0 effective 06/21/2013 to 01/11/2017