Polymerase chain reaction based detection of Mycobacterium tuberculosis in tissues showing granulomatous inflammation without demonstrable acid-fast bacilli

Background Cutaneous tuberculosis is especially difficult to distinguish from other granulomatous dermatoses. We used polymerase chain reaction (PCR) to evaluate the incidence of cutaneous tuberculosis and atypical mycobacterial infection in formalin-fixed, paraffin-embedded tissues with unspecified granulomatous inflammation and negative results for acid-fast bacilli (AFB), and analyzed the pattern of cutaneous tuberculosis in this group of patients. Methods A total of 38 specimens which had been collected from 36 patients and fulfillled the criteria for tissues described above were used in this study. Two different primer pairs targeting the gene encoding for 16S ribosomal RNA (common to all mycobacteria) and the insertion sequence IS6110 (specific for M. tuberculosis complex) were used in the PCR assays. The clinical characteristics, histopathologic findings, and culture results of the patients were also analyzed. Results Four specimens were excluded from the analysis due to the lack of internal control testing. Of the remaining 34 specimens, 22 were PCR positive for the 16S rRNA gene. Among them, 18 specimens were PCR positive for both the 16S rRNA gene and IS6110. Cutaneous tuberculosis could be diagnosed in these 18 cases (56.2%). Out of the 18 cases, there were 8 women and 10 men. The age range was 15-77 years (mean: 44.2 years). After reviewing their clinical presentation, 11 cases were considered as tuberculosis verrucosa cutis, 6 cases as lupus vulgaris, and 1 case as erythema induratum. The remaining 4 cases (12.5%) positive only for 16S rRNA gene were considered as possible atypical mycobacteria infection. Conclusions These results show that in paucibacillary form of cutaneous tuberculosis with unclassical clinical and histological presentation, this PCR system provides rapid and sensitive detection of M. tuberculosis DNA in formalin-fixed, paraffin-embedded specimens. Cutaneous tuberculosis represents a significant proportion in specimens showing granulomatous inflammation. In areas like Taiwan, where prevalence of pulmonary tuberculosis is still high, tuberculosis verrucosa cutis and lupus vulgaris are common forms of cutaneous tuberculosis and are seen more frequently than atypical mycobacterial infection.