Detection of mycobacterial DNA in the skin. Etiologic insights and diagnostic perspectives.

BACKGROUND: Tuberculosis may be as old as mankind and continues to be a serious medical problem today. Cutaneous tuberculosis shows considerable morphological variability, and it is included in the differential diagnosis of many other skin disorders. It is especially difficult to distinguish skin tuberculosis from other granulomatous processes of the skin. Therefore, reliable laboratory tests are needed to confirm or rule out the diagnosis. However, the diagnostic identification of Mycobacterium tuberculosis and related organisms has remained difficult using conventional laboratory tests (ie, microscopy and culture). OBSERVATIONS: The diagnostic usefulness of molecular techniques, especially the polymerase chain reaction (PCR), in skin tuberculosis is reviewed, and the technical issues of PCR in general are discussed, with special regard to the analysis of mycobacterial DNA in skin specimens. The PCR has been successfully applied to detect DNA from M tuberculosis in lupus vulgaris and several other forms of skin tuberculosis. It has also been used to identify mycobacterial DNA in certain forms of tuberculids, thereby supporting the long- and often-debated tuberculous origin of these skin disorders. Investigations of the presence of mycobacterial DNA in cutaneous sarcoidosis have not lent support to a general role for mycobacteria in sarcoidosis.
CONCLUSIONS: Polymerase chain reaction-based detection of M tuberculosis DNA in skin samples may extend and improve the diagnostic panel for cutaneous tuberculosis, if the technique is prudently and properly used. Furthermore, PCR provides exciting opportunities to gain further insight into the pathogenesis of cutaneous tuberculosis and other granulomatous skin diseases.