C-208. Inhibition Testing among Participants in CDC’s Model Performance Evaluation Program for Nucleic Acid Amplification for Mycobacterium tuberculosis, 2004-2007

L. O. Williams1, D. M. Warshauer2, P. Robinson1, J. Nichols2, J. Tans-Kersten2;
1CDC, Atlanta, GA, 2Wisconsin State Lab. of Hygiene, Madison, WI.

CDC conducts a voluntary model performance evaluation (MPEP) program for M. tuberculosis nucleic acid amplification testing (NAA-TB). The program provides external quality assessment for U.S. and non-U.S. laboratories performing NAA-TB tests by sending challenge samples to MPEP participants every six months. Participants include hospital, public health, and independent laboratories. One goal of the NAA-TB MPEP is to provide information which will encourage optimal laboratory performance and practices. The current CDC guidelines for NAA-TB testing include recommendations for testing for the presence of inhibitory substances in NAA-TB-negative specimens. (MMWR 2000; 49:593-594) Inhibition testing on NAA-TB-negative specimens is the only way to distinguish between true negative results and results that are falsely-negative due to inhibitors. From 2004 to 2007 three specimens containing inhibitors (high molarity phosphate and heparin) were included in the challenge samples. In January 2004, 29/89 (32.6%) of laboratories responded correctly (“inhibitors detected”) for the inhibited specimen. In June 2006, 50/82 (61.0%) responded correctly, and in June of 2007, the correct response rate increased to 58/81 (71.6%). Additionally, participants were asked if they performed inhibition testing on NAA-TB-negative patient specimens. In January 2004, 49/84 (58.3%) stated they performed inhibition testing compared with 69.4% (59/85) in June 2006, and 63/82 (76.8%) in June 2007. Over the 4 year period there was a significant improvement in the number of laboratories correctly identifying inhibitory samples in the MPEP program and in those stating that they followed the CDC guidelines to test for the presence of inhibitors in NAA-TB-negative specimens.