Tuberculosis — Written Clinical Reasoning

Model answer — Part A: the anti-TNF candidate with a positive IGRA

Frame the problem. This is the highest-stakes latent-TB scenario in adult medicine: TNF blockade in a sensitised host reactivates TB rapidly and often in disseminated form — the signal was clear within infliximab's first years of use ^[1]. The consultation has two sequential questions: does he have active disease now, and if not, how do I prevent reactivation while his Crohn's is treated ^[2].

Assessment. Take a symptom history actively rather than passively — cough, fevers, night sweats, weight loss, haemoptysis — and a risk history: BCG status, prior TB contact, prior treatment, and other immunosuppression. Examine for lymphadenopathy and chest signs. His imaging is the pivot: fibronodular apical scarring stable over 3 years favours old, contained disease, but stability on a plain film does not exclude low-grade activity ^[2].

Distinguishing latent from active is discriminator examiner is scoring. Neither IGRA nor TST can do it — both measure immune memory, not bacilli, and their positive predictive value for progression is only a few per cent even in high-risk cohorts ^[4]. The exclusion of active disease is therefore clinical plus microbiological: in an asymptomatic man with stable imaging, obtain sputum for smear, NAAT and culture if there is any symptom, any new radiological change, or any doubt — and accept that a well patient with stable films and no organisms is latently infected ^[7].

Management. Treat latent infection before the biologic: 4 months of daily rifampicin is the preferred regimen — non-inferior to 9 months of isoniazid, with better completion and less hepatotoxicity ^[3]. Reconcile rifampicin's interactions with his other drugs. Infliximab can start after at least a month of preventive therapy is on board when the Crohn's cannot wait; document baseline LFTs and counsel about orange urine from rifampicin. State clearly what you will NOT do: start infliximab with untreated latent TB, or give preventive monotherapy while active disease remains unexcluded ^{[1] [2]}.