Comparison of the mouse foot pad test with a Buddemeyer type radiorespirometric assay in detecting viable Mycobacterium lepra  e in human lesional biopsies

Aim : This study assesses the utility of a Buddemeyer type radiorespirometric (RR) assay in detecting viable Mycobacterium leprae in clinical samples taking the mouse foot pad (MFP) test as gold standard.
Methods : A total of 131 skin biopsy specimens comprising of 56 untreated, smear-positive, borderline lepromatous and lepromatous leprosy (BL-LL) and 75 untreated, smear-negative, borderline tuberculoid and mid-borderline (BT-BB) specimens were processed by both the methods. The cut-off value (in counts per minute, cpm) for test samples in the RR assay was determined using nonleprous (normal) skin biopsy specimens.
Results : In the untreated BL-LL and BT-BB groups, 86 and 56% of the cases tested positive in the RR assay respectively, which was comparable to the results of the MFP test (80 and 45% respectively). The overall concordance between the two tests was 74.79%, whereas the sensitivity and specificity were 75 and 74.3% respectively. A Kappa value of 0.459 indicated a fairly good agreement between the two methods. There was no linear relationship observed between the bacteriological index (BI) and the morphological index (MI) with the counts per minute (cpm) output.
Conclusion : The results indicate the ability of this RR assay to detect viability; however a comparison with another sensitive method would further validate the assay system.

Keywords
Mycobacterium leprae, Mouse foot pad test, Radiorespirometric assay, Viability