Fasting and Warfarin

Document Type : Letter to the Editor


1 Department of Pharmacy, Singapore General Hospital, Singapore

2 Department of Haematology, Singapore General Hospital, Singapore


Recently, we reported the findings of a study on the effects of Ramadan fasting on a group of 32 Muslim patients taking warfarin.13 Weekly INR (international normalized ratio) readings were taken from subjects with previously stable INRs over a three-month period spanning pre-Ramadan to post-Ramadan. We found a statistically significant increase in the mean INR by 0.23 (p=0.006) during Ramadan from the pre-Ramadan month and decreased by 0.28 (p<0.001) after Ramadan. There was no significant difference (p=1.000) in mean INR between the non-Ramadan months. Importantly, there was a decline in the time within therapeutic range (TTR) during Ramadan with a corresponding increase in TTR above the therapeutic target range. %TTR declined from 80.99% before Ramadan to 69.56% during Ramadan (p=0.453). The first out-of-range INR was seen around 12.1 days (95% CI 9.0-15.1) after the start of fasting and returned within range about 10.8 days (95% CI 7.9-13.7) after Ramadan. Time above range increased from 10.80% pre-Ramadan to 29.87% during Ramadan (p=0.027), while time below range increased from 0.57% during Ramadan to 15.49% post-Ramadan (p=0.006). This observation is directly attributable to the effects of fasting.


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