TB rifampicin dosing — DTM&H MCQ

Correct answer: A — 600 mg daily

The correct answer is A, 600 mg daily. In UK TB treatment protocols (based on NICE NG33 and BNF guidance), rifampicin is dosed at approximately 10 mg/kg once daily, with adults weighing over 50 kg receiving 600 mg daily and those under 50 kg receiving 450 mg daily. This patient weighs 65 kg, placing him firmly in the greater-than-50-kg band, so 600 mg daily is the correct dose for both the intensive (initial) phase and the continuation phase, as rifampicin dosing does not change between phases (only the companion drugs pyrazinamide and ethambutol are dropped after the first 2 months). The 10 mg/kg target in a 65 kg man equates to 650 mg, but the standard fixed dose of 600 mg is used rather than titrating exactly to weight, reflecting the practical banding used in UK combination tablets and prescribing guidance. Why the other options are wrong: E. 300 mg daily: this is subtherapeutic and corresponds to roughly half the correct weight-band dose; it would risk treatment failure and selection of rifampicin resistance. D. 450 mg daily: this is the correct dose only for adults weighing less than 50 kg, not for a 65 kg patient. B. 150 mg daily: this dose is far too low for treatment and is not a recognised adult TB dosing tier; it would not achieve therapeutic levels. C. 900 mg daily: this exceeds standard weight-banded dosing and increases the risk of hepatotoxicity and other dose-related adverse effects without added efficacy in a 65 kg patient. Key point: rifampicin dosing in UK TB regimens is weight-banded at approximately 10 mg/kg, giving 450 mg daily if under 50 kg and 600 mg daily if 50 kg or over, and this dose is the same throughout both the intensive and continuation phases.

Reference: NICE Guideline NG33, Tuberculosis (2016, updated 2019); BNF, Rifampicin, medicines.org.uk