The possibilities here are that the patient had ITB from the onset which was misdiagnosed as CD, or that the patient contracted ITB during immunosuppression with infliximab. There are numerous case reports in the literature where ITB was misdiagnosed as CD. There are also many reports of patients on infliximab developing TB, but none of these patients had TB isolated to the intestine. The most likely explanation in this patient is that of ITB misdiagnosed as CD. This is given the possibility of false negative skin test and TB PCR at initial evaluation, poor response to successive immunosuppression therapy, further worsening of bowel symptoms with infliximab therapy needing surgery and the complete resolution of clinical and endoscopic features with a course of ATT.
Histology is one of the most definitive methods of differentiation of CD from ITB. Features described to be only found in ITB are caseating granulomata and confluent granulomata. Large numbers of granulomata (≥10 per biopsy site), large granulomata (≥0.05 mm), ulcers lined by epithelioid histiocytes and disproportionate submucosal inflammation are other features suggested to support ITB. Acid-fast bacilli in biopsy specimens, although very specific, are not frequently encountered .
Polymerase chain reaction of biopsy specimens for mycobacterial DNA is rapid and reliable but still has false positives and negatives. There has been suggestion that faecal PCR is more sensitive than tissue PCR, but this needs to be investigated further . Culture for M. tuberculosis from biopsy although definitive, is low in sensitivity (50 %) . Specimens for TB culture was not taken at colonoscopy from this patient.
Although positivity of a tuberculin skin test (TST) and interferon gamma-release assay (IGRA) is supportive of ITB, in areas with a high prevalence of TB this may be an incidental positivity due to past TB exposure or in the case of TST also non-tuberculous mycobacteria or BCG vaccination. Furthermore, the use of corticosteroids and immunosuppressive drugs in a patient with IBD may limit the diagnostic utility of both these tests .
When all else fail, clinicians have a tendency to treat for ITB and monitor the response. This approach is safe given the potential for life threatening flare up of ITB, if treated as for CD in the setting of missed ITB, especially with immunosuppressive agents and biologics. Clinical studies which have assessed this approach have suggested that a short term (2–3 month) anti-TB treatment trial followed by re-endoscopy to look for healing may be valuable in the differential diagnosis of difficult cases . Fortunately for this patient despite use of steroid, azathioprine, 6-MP and infliximab, there was no life threatening systemic flare up of tuberculosis.