Falsely elevated international INRs

Q: What is the most common cause of falsely elevated international normalized ratio (INR)?

Answer: Heparin in the blood sample

The two major causes of falsely elevated INRs in ICU are:

1. The presence of heparin in the drawn blood sample, particularly in hospitalized patients, is the most common cause of falsely elevated INRs. It is important to draw blood samples from a peripheral vein. Drawing blood samples from central venous catheters (CVCs) or arterial lines (A-lines) in ICU can give false information. 

2. Second major cause of falsely elevated INRs is drawing less than the optimum quantity of blood. This provides a higher than normal ratio of citrate anticoagulant drawn plasma. 

Also, the presence of a lupus anticoagulant artificially prolongs the INR value.

#hematology

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References:

1. Froom P, Barak M. Falsely elevated prothrombin international normalized ratio values. Am J Clin Pathol. 2010 Jul;134(1):86-9. doi: 10.1309/AJCPDKQV2UMO2FAB. PMID: 20551271.

2. Delate T, Witt DM, Jones JR, Bhardwaja B, Senser M. Falsely elevated international normalized ratio values in patients undergoing anticoagulation therapy: a descriptive evaluation. Chest. 2007 Mar;131(3):816-822. doi: 10.1378/chest.06-2200. PMID: 17356098.