Prothrombin Time with International Normalized Ratio (INR)

Additional Information:


  • INR
  • Pro Time
  • Protime
  • Protime, PT/INR
  • PT/International normalized ratio

UFHPL Epic order code: LAB320

UFHPL – Core Laboratory Recommendations for Anticoagulant Therapy

Although the prothrombin time (PT) and activated thromboplastin time (aPTT) are both affected by vitamin K antagonists (Coumadin) and by heparin, the PT/INR is the recommended first-line assessment of Coumadin therapy.

As an alternative to aPTT measurements for unfractionated heparin, a direct heparin assay can be performed. Heparin is measured as anti-Xa units. Indeed, this is the recommended modality of assessing low molecular weight heparin therapy, since the aPTT is a poor indicator of the response to LMW heparin.

There is increasing use of direct thrombin inhibitors, such as Argatroban. We recommend monitoring the latter with the aPTT using a prolongation of approximately 1.5 to 2.5 times the baseline.

Anticoagulation therapy monitoring should be done in careful consultation with the UF Health Pharmacy and the UF Department of Medicine, Division of Hematology.

CPT Code(s):


Specimen Requirements:

Type: Plasma, sodium citrate, platelet-poor

Container/Tube: Light blue (3.2 percent sodium citrate)

Sample Volume: 2 mL of platelet-poor plasma (0.5 mL minimum)


  • Ambient: 24 hours
  • Frozen: 1 month
Unacceptable Conditions:

  • Clotted or hemolyzed samples
  • EDTA plasma
  • Serum
Specimen Retention Time: 5 days


This test is used to monitor the intensity of oral anticoagulant therapy when combined with INR reporting; as a screening assay to detect deficiencies of one or more coagulation factors (factors I, II, V, VII and X); and as a screening assay to detect coagulation inhibition (circulating anticoagulants).


Electromagnetic endpoint detection


Within 24 hours

Reference Values:

11.6 – 14.1 seconds