Impact of Antibacterials on the Quality of Anticoagulation Control in Patients Initiating Warfarin Therapy

Warfarin interacts with antibacterials to prolong the prothrombin time international normalized ratio (PT-INR) and increase the risk of bleeding. Patients initiating warfarin therapy often undergo precise dosage adjustments; however, the clinical implications of these interactions with antibacterials remain unclear. This study aimed to clarify the effect of antibacterials on PT-INR during the warfarin induction phase.

This was a retrospective, observational study. Patients who were newly treated with warfarin after cardiovascular surgery were included. The primary endpoint was the comparison of the maximum PT-INR and time in therapeutic range (TTR) after warfarin initiation between the antibacterial-treated (ABx) and non-treated (non-ABx) groups.

The maximum PT-INR was significantly higher in the ABx group (which included β-lactams, glycopeptides, quinolones, tetracyclines, and aminoglycosides) than in the non-ABx group (median [interquartile range] 2.37 [2.03–2.71] vs. 2.08 [1.93–2.33]; P = 0.005); however, the TTR did not differ significantly (65% [44–76] vs. 71% [43–85]; P = 0.150). The odds ratio for maximum PT-INR > 2.6 with antimicrobial therapy was 2.51 (95% confidence interval 1.21–5.21).

Antibacterial therapy was a risk factor for a maximum PT-INR >2.6. However, there was no association with the TTR, which is a marker of good outcomes. This was due to the strict warfarin dosing regimen according to the algorithm, which immediately and appropriately adjusted for PT-INR overexpansion.

Antibacterials have been suggested to increase PT-INR during the induction phase of warfarin. However, with strict dose adjustments, the clinical impact on the PT-INR and TTR is likely limited.