Anti-tuberculosis IgG antibodies as a marker of active mycobacterium tuberculosis disease

Anti-Mycobacterium tuberculosis IgG antibodies may aid in the diagnosis of active M. tuberculosis disease. We studied whether anti-M. tuberculosis IgG antibodies are elevated in active M. tuberculosis disease and assessed factors contributing to false positive and negative results. A retrospective study of 2,150 individuals tested by the QuantiFERON-TB Gold In-Tube (QFT-GIT) assay was conducted at University of Utah, ARUP Laboratories, November 2008 to December 2010. All samples were tested with the InBios Active TbDetect™anti-TB IgG antibody assay. Of 1,044 patients with a positive QFT-GIT, 59 (5.7%) were positive for M. tuberculosis antibodies. Fourteen of 1,106 (1.3%) with a negative or indeterminate QFT-GIT were positive for M. tuberculosis antibodies. M. tuberculosis antibody tests were positive in 61.5% with confirmed active M. tuberculosis disease and other mycobacterial infections. Over half of the false negative M. tuberculosis antibody tests occurred in patients ≥ 90 years of age. False positives were seen in 12.9% of autoimmune patients. The odds ratio of being 36 positive on the QFT-GIT and the InBios TB IgG assay increased with confirmed M. tuberculosis disease or highly suspected M. tuberculosis disease and was 86.7 (95% 38 confidence interval [CI], 34.4-218.5) in these two groups when compared to patients negative on both tests. Although anti-M. tuberculosis antibodies can be detected in patients with active M. tuberculosis disease, caution should be used in patients where immunoglobulin levels may be decreased or in patients with autoantibodies.