Cost-Effectiveness of Blood Agar for Isolation of Mycobacteria

Abstract

Mycobacterium species are grown using specific media that increase laboratory cost, thus hampering their diffusion in resource-limited countries. Preliminary data suggested that versatile blood agar may be also used for mycobacterial culture. We examined the growth of 41 different Mycobacterium species on 5% blood agar. Over a 24-month period we analysed isolation of mycobacteria after parallel inoculation of clinical specimens into both a reference automated system (BACTEC 9000 MB broth) and 5% blood agar slant tubes, after NaOH decontamination, and compared the cost of performing 1,000 analyses using these two techniques. Mycobacterium reference species cultured on blood agar, with the exception of Mycobacterium ulcerans. Inoculation of 1,634 specimens yielded 95 Mycobacterium isolates. Blood agar performed significantly more efficiently than BACTEC 9000 MB broth (94 vs 88 isolates, P = 0.03). Decontamination of Candida albicans in 5 specimens by addition of amphotericin B in blood agar yielded one more M. tuberculosis isolate that could not be isolated in BACTEC broth. Uneven distribution of time to culture positivity for M. tuberculosis had a median (range) of 19±5 days using blood agar and 26±6 days using BACTEC 9000 MB broth. Cost for 1,000 analyses in France was estimated to be of 1,913 euros using the blood agar method and 8,990 euros using the BACTEC 9000 MB method. Blood agar should be regarded as a first-line medium for culturing Mycobacterium species. It saves time, is cost-effective, is more sensitive than, and at least as rapid as the automated method. This is of particular importance for resource-limited countries in which the prevalence of tuberculosis is high. Mycobacteria are organisms responsible for animal and human infections comprising tuberculosis due to Mycobacterium tuberculosis and other opportunistic infections. Such infections require specific antibiotic treatment and prevention of secondary cases in the occurrence of pulmonary tuberculosis. The accurate diagnosis of mycobacteria infection is therefore of prime importance. Isolation and culture of mycobacteria from diseased clinical specimens is the gold standard for diagnosis. It relied for decades on the use of specific isolation media, resulting in most laboratories not attempting such diagnosis. Alternatively, specific automates and culture broths are available only in developed countries. We herein demonstrate that blood agar, a basic medium widely and routinely used in laboratories worldwide, is suitable for the isolation and culture of mycobacteria encountered in human pathology, including tuberculosis. It performed at least as well as reference culture broth. Morever, using blood agar was cost-effective. Blood agar should be recommended as a routine medium for the isolation of most pathogenic organisms, including mycobacteria, both in developing and developed countries.