Retrospective diagnosis of lymphatic tuberculosis in frozen samples using two genetic amplification methods, Xpert® MTB/RIF ULTRA and Abbott RealTime MTB Assay

The main objective of the present study is to assess the sensitivity and specificity of a retrospective diagnostic of lymphatic tuberculosis (LTB), testing frozen samples using gene amplification PCR methods. The secondary objective was to compare the results of two different commercial tuberculosis gene amplification methods for this purpose.

We retrospectively studied 38 frozen samples, previously processed for mycobacterial culture between January 2014 and August 2019. The results of the previous cultures were: 21 samples positive for Mycobacterium tuberculosis complex (MTB) (5 being smear positive), 7 samples culture positive for Mycobacterium avium-intracellulare complex and 10 samples which were mycobacterial culture negative and discarded for LTB diagnosis, used as controls. The samples were processed using two gene amplification methods: Xpert® MTB/RIF Ultra (Cepheid) and Abbott RealTime MTB Assay (Abbott).

Compared to initial culture results the sensitivity and specificity of Xpert® MTB/RIF Ultra were 57.1% and 100% and 52.3 % and 92.5%, respectively for the Abbott RealTime MTB assay. The differences were not statiscally significant. In addition, there were no differences according to the period of freezing.

Gene amplification of frozen samples confirmed the diagnosis of lymphatic TB in almost 60% of cases, allowing retrospective diagnosis in initially non suspected cases. Both gene amplification techniques tested were equally useful.