Association between a favourable clinical response to anti-tuberculosis treatment and a positive PCR test for mycobacterium tuberculosis in histologically proven erythema induratum/nodular vasculitis

Abstract

Background: Polymerase chain reaction (PCR) has been used for many years to detect Mycobacterium tuberculosis (M.tb) DNA in biopsy tissue of erythema induratum (EI)/nodular vasculitis. Studies to ascertain the association between a positive PCR and clinical response to anti-tuberculosis (TB) therapy are lacking. Objectives: Our aim was to determine the association between a favourable clinical response to anti-TB treatment and a positive PCR for M.tb in histologically proven EI. Methods: Twenty-four cases of histologically proven EI were identified that had been biopsied in our department between 1 January 2009 and 31 December 2014. The response to anti-TB therapy was then determined retrospectively, establishing in which patients the subcutaneous nodules of EI had resolved on treatment. Thereafter the formalin-fixed paraffin-embedded tissue sections were sent for PCR. Results: All patients included in our study received anti-TB treatment. The clinical response was favourable in eighteen patients (75%), no response to treatment was observed in five (20,8%) and in one patient (4,1%), who was lost to follow-up, the response could not be determined. The PCR for M.tb was positive in only one sample (4,1%) whereas the other twenty-three samples (95,8%) had a negative PCR for M.tb. Conclusion: The PCR technique on formalin-fixed tissue remains subject to multiple technical pitfalls. Thus only positive results are meaningful, whilst negative results are inconclusive.