False-positive Xpert® MTB/RIF assays in previously treated patients: need for caution in interpreting results.

Xpert(®) MTB/RIF is the initial diagnostic test of choice for tuberculosis (TB). It is not known if false-positive results are more common in previously treated patients. We report four patients with successful treatment for TB up to 5 years previously who presented with respiratory tract infection and were Xpert-positive, but had negative TB cultures and clinical improvement without anti-tuberculosis treatment. We hypothesise that the Xpert results were false-positive due to the presence of dead Mycobacterium tuberculosis bacilli in lungs and sputum. Further work is required to determine the specificity of Xpert in previously treated patients.