A diabetic female on INH and rifampicin for TB developed DVT.She was staed on warfarin, PT is not raised, and next step should be:
**Core Concept**
The patient's presentation is a classic example of drug-drug interaction between rifampicin and warfarin, leading to decreased warfarin efficacy. This interaction is due to rifampicin's induction of the cytochrome P450 enzyme system in the liver, which accelerates the metabolism of warfarin.
**Why the Correct Answer is Right**
Warfarin acts by inhibiting vitamin K-dependent clotting factors, and its anticoagulant effect is measured by the prothrombin time (PT) test. However, rifampicin's induction of the cytochrome P450 system leads to increased metabolism of warfarin, resulting in decreased warfarin levels and a failure to adequately inhibit clotting factor synthesis. As a result, PT may not be raised despite the presence of a DVT.
**Why Each Wrong Option is Incorrect**
**Option A:** Administering a higher dose of warfarin would not be effective, as rifampicin's induction of the cytochrome P450 system would continue to accelerate warfarin metabolism.
**Option B:** Switching to a different anticoagulant, such as heparin, may be necessary, but it does not address the underlying issue of decreased warfarin efficacy.
**Option C:** Monitoring PT is crucial, but in this case, PT may not be raised due to the drug-drug interaction.
**Clinical Pearl / High-Yield Fact**
Rifampicin is a potent inducer of the cytochrome P450 enzyme system, and its use can lead to decreased efficacy of many medications, including warfarin, phenytoin, and theophylline.
**Correct Answer: D.**