Avaliação de modificações no método de redução de nitrato visando a detecção de Mycobacterium tuberculosis multirresistente

Abstract

Tuberculosis (Tb) is an infectious disease of international distribution. Problems in the diagnosis, as time requirement and high costs for implementation in public health care, increases the number of the disease cases. Specifically about the most important antibiogram assays, the “nitrate reduction assay” (MRN) provides faster results (7 to 14 days) than the “proportion method (MP)” (28 days), however this last one is considered as “gold standard” by the World Health Organization. The aim of this work was to evaluate modification in the method of nitrate reduction (MRN), used to check the sensitivity of M. tuberculosis strains to primary drugs administered for the treatment of TB, as an alternative to reduce costs and to simplify this methodology, in order carry out it in the primary health care to verify the resistance of TB. It was evaluated the accuracy (2X2 table) and agreement (observed agreement and Kappa) between the MP and six variations of NRM: two sources of nitrate (NaNO3 and KNO3); and three different media for solid cultivation (Middlebrook 7H10, Löwenstein-Jensen and Ogawa). The drugs evaluated were: isoniazid (INH), rifampicin (RIF), streptomycin (STR) and ethambutol (EMB); and the tests were conducted with: a strain of Mycobacterium tuberculosis H37Rv (standard sensitivity), four resistant strains (different patterns of resistance to each drug) and twenty-four clinical isolates with unknown antibiotic susceptibility results. Under the studied conditions, excellent reproducibility and agreement was obtained between the original NRM and the MP, confirming that the original NRM is an alternative to antibiogram assays. In the assays carried out with modified MRN (potassium nitrate was substituted by sodium nitrate) it was obtained faster results and full agreement comparing with the results of the MP and the original MRN, indicating that this modified medium could be used in antibiogram assays in the basic care health, since sodium nitrate is a nitrogen source cheaper and more easily obtained. The result of the assays using STR, in all variations of the MRN, indicates that are necessary further evaluation, especially using OG medium. The Middlebrook medium presented recurrent contamination making impossible to carry out analysis.