The tuberculin test can be helpful in the diagnosis of Mycobacterium tuberculosis infection. A recent CMAJ article mentions that false-positive test results can occur in those who have received BCG (bacille Calmette-Guérin) vaccination in childhood1 but that positive reactions usually wane over time. No mention is made of the effect on the tuberculin test of intravesicular BCG therapy.
Patients with a positive tuberculin test reaction are usually asked if they have a history of BCG vaccination; however, most physicians do not ask their elderly patients if they have a history of intravesicular BCG therapy, which is commonly used for bladder cancer.2 The following case illustrates how careful review of a patient's history and medical records confirmed a false-positive reaction.
A 74-year-old man presented with a 9-kg weight loss. A chest x-ray film revealed fibrotic changes in the left lower lobe. A Mantoux test was strongly positive with induration of 18 mm at 48 hours. The patient denied constitutional symptoms of fever, chills or night sweats. He had a mild dry cough. He denied hemoptysis. He had been smoking for 60 years and had a history of emphysema and throat and bladder cancer. There was no history of exposure to tuberculosis, and he did not recall having a BCG vaccination in childhood. He had a negative tuberculin skin test in 1995. Chest CT showed old granulomatous changes and emphysema. The possibility of active tuberculosis was entertained. Findings on the chest x-ray film were unchanged from 1991. Urine and sputum cultures for acid-fast bacilli were negative. Isoniazid prophylaxis was considered in view of induration of 15 mm or greater and recent conversion.
The patient did not know what treatment he had received for bladder cancer other than that it was some form of "chemotherapy," but hospital records confirmed that he received intravesicular BCG immunotherapy in 1996. The recent tubercular conversion was concluded to be false positive and isoniazid prophylaxis was avoided.
Among patients given intravesicular BCG treatment for superficial bladder cancer, up to 65% may have a positive tuberculin skin test reaction.2 For patients with a positive reaction who have a history of bladder cancer, physicians should investigate whether they have had intravesicular BCG therapy before they commit to isoniazid prophylaxis.