A woman in her 80s had a hospital acquired pneumonia (HAP). During this time, her INRs did the following whilst her dose of warfarin was maintained at 5mg OD (except for a reduction to 4mg on Day 5):
| Day 1 of HAP | Day 4 | Day 5 | Day 6 | |
| INR | 1.8 | 2.5 | 4.7 | 6.2 |
| Albumin | 36 | 29 | 24 | 20 |
She was started on tazocin and clarithromycin on Day 1 of the HAP too.
Why did the INR rise so dramatically?