Pathergy as a Cause of False-positive Tuberculin Skin Test

Abstract

To the Editors: Differential diagnosis of intestinal tuberculosis (ITB) versus inflammatory bowel conditions can be challenging, especially in areas in which tuberculosis prevalence is very high or very low.1 Clinical presentation can be indistinguishable, and the extraintestinal manifestations of both conditions can be similar. Correct and prompt diagnosis is critical to avoid dire consequences, especially in the era of biologic therapies.2 The diagnosis of ITB can be very difficult, as both acid-fast bacilli staining and biopsy culture have low sensitivity, and the new polymerase chain reaction assays are not available in poor resource settings.1 Tuberculin skin test (TST) is often used in the diagnostic process. We report on a seldom considered cause of false-positive results to TST, possibly leading to a wrong diagnosis. A 15-year-old, Indian-born girl was admitted to our hospital because of a 6-month history of diarrhea and abdominal pain. She also presented recurrent painful oral ulcers, and ocular redness and pain in the left eye. One year earlier, she had received a diagnosis of tuberculosis-related erythema nodosum. A TST performed on that occasion resulted in extensive induration followed by blistering and ulceration of a 3-cm-wide skin area. Chest radiograph was negative for tuberculosis lesions, and she had been treated with a 6-month course of isoniazid. On admission to our hospital, laboratory tests showed elevated erythrocyte sedimentation rate, 78 mm/h, and fecal calprotectin (74 mg/kg of stools; normal values, <15), a marker of intestinal inflammation. Colonoscopy and video capsule endoscopy were performed to rule out ITB and inflammatory bowel disease. Sparse aphthous lesions were seen in the distal ileum. Polymerase chain reaction assay for Mycobacterium tuberculosis and staining for acid-fast bacilli were negative, as were also chest radiograph and interferon-gamma release assay results (QuantiFERON-TB Gold test; Cellestis Limited, Australia). An ophthalmology examination revealed acute anterior uveitis in the left eye. The patient's findings fulfilled the criteria for Behçet disease (BD) according to the International Study Group (recurrent oral ulceration, uveitis, erythema nodosum).3 Intestinal involvement in the form of ileal mucosal ulcers is also commonly reported in BD.4 She was treated with a 2-month course of oral prednisone and made a full recovery. The strongly positive TST can be explained as a pathergic reaction. Pathergy is characterized by the triggering of a cutaneous inflammatory response to minor trauma like needle prick. It is mainly described in BD and in some other neutrophilic dermatoses like pyoderma gangrenosum.5 Therefore, in presence of clinical and endoscopic finding compatible with both ITB and BD, the possibility of a false-positive TST caused by a pathergic reaction should be considered. On the other hand, BD and tuberculosis can coexist, so caution is mandatory. Serena Pastore, MD Samuele Naviglio, MD Alessandro Ventura, MD Institute for Maternal and Child Health IRCCS “Burlo Garofolo” University of Trieste Trieste, Italy