Role of Immunocytochemistry in Detection of Mycobacterium tuberculosis Antigen by Fine Needle Aspiration Cytology.

Background: Detection of extrapulmonary tuberculosis (EPTB) by Ziehl-Neelsen (Z-N) staining in fine needle aspirates is challenging due to the low yield of acid-fast bacteria (AFB). Mycobacterial culture, the gold standard, takes 4-8 weeks. Polymerase chain reaction with 100% sensitivity and 92.1% specificity is expensive. Mycobacterial antigens produced by the tubercle bacilli consist of several proteins and enzymes. The protein purified from Mycobacterium tuberculosis is called MPT. The MPT64, a 24-kd protein, has not been detected in non-tuberculous mycobacteria. We aim to study the role of immunocytochemical (ICC) in the detection of EPTB in fine needle aspiration cytology materials by using MPT64 antibody and compare it with culture and Z-N staining, as ICC is not routinely practiced for diagnosing EPTB.

Materials and methods: A total of 134 patients having enlarged nodes with suspected EPTB were included; however, only 96/134 cases were suitable for statistical analysis. Papanicolaou, May-Grünwald-Giemsa, Z-N staining, ICC, and mycobacteria culture were performed.

Results: AFB was positive in 16%, 22.9% of culture-positive, and 11% of MPT64 positive. The sensitivity and specificity of ICC compared to mycobacterial culture were 45.4% and 99%, respectively. ICC and culture had a moderate agreement with the Kappa value of 0.535. The positive and negative predictive values of ICC with culture were 91% and 86%, respectively.

Conclusion: This study tried to improve the technique's sensitivity to facilitate its use in routine laboratory practice. Nonetheless, our results showed no significant improvement over the currently popular Z-N stain, although a comparison between the ICC technique performed on smears and cell block sections showed better results in the latter.