Pre-analytical variables which can affect PT-INR results?
A patient of mine once tested his protime INR results (Yes, he is a warfarin user) sending blood samples to 2 tertiary hospitals and 2 secondary laboratories. Upon receiving his results he was dumbfounded to get different results (ranging from 1.5 up to 3.0). So i decided to research on the conditions affecting PT-INR results. So here's what i got:
Pre-analytical variables (accounts to 64% of all errors in PT/INR testing)
1. Anticoagulant used - WHO guidelines recommends 3.2% buffered citrate.
2. The test tube must be completely filled up with blood (at least 90% full) for proper anticoagulant to blood ratio.
3. The specimen should be centrifuged for at least 10 minutes to create a platelet poor plasma (platelets tends to normalize INR in patients taking warfarin)
4. For delayed testing (> 24 hours) the platelet poor plasma should be frozen.
Analytical variables - The American Association of Clinical Chemistry revealed that there is a poor agreement among PT INR results depending on the method used by the laboratory ( Juha et al Poor aAgreement among PT INR: comparison of Seven Commercial Reagents Clin Chem 51: 533-560,2005). Hence, the take home message for med tech is to know the pre analytical variables and to MDs know the method of your hospital. Recently the AHA advocates the use of self test meters for INR monitoring (Coaguchek by Roche) for patients taking warfarin, its accurate, its reliable and above all it provides instant result to avoid over dosage of warfarin.
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