Ancillary molecular testing on tissue is available for mycobacterial disease; however, judicious use of highly sensitive tests, such as polymerase chain reaction (PCR) and sequencing, should be guided by histologic parameters. We sought to investigate the utility of performing acid-fast stains (AFS) on skin biopsy specimens with granulomatous inflammation without an otherwise obvious histologic or clinical explanation.
Our retrospective review identified 31 patients with biopsy specimens showing granulomatous inflammation that had simultaneous AFS and mycobacterial culture or PCR performed.
Biopsy specimens from eight (25.8%) patients had AFS interpreted as positive or suspicious for acid-fast bacilli. Eight had positive cultures and one had positive PCR. One biopsy specimen with AFS that showed occasional acid-fast structures that were interpreted as "suspicious" for mycobacteria was associated with a negative culture and negative PCR. Three (9.7%) biopsy specimens with negative AFS had positive cultures, and 19 (61.3%) biopsy specimens with negative AFS also had negative culture results. In our biopsy specimens, sensitivity of AFS was 72.7% and specificity was 95.0%. Positive predictive value of AFS was 88.9%, and negative predictive value was 86.4%.
AFS has good sensitivity and excellent specificity and should be performed on all unexplained granulomatous tissue reactions of skin in conjunction with mycobacterial culture.