Cord Formation in MB/BacT Medium Is a Reliable Criterion for Presumptive Identification of Mycobacterium tuberculosis Complex in Laboratories with High Prevalence of M. tuberculosis

Abstract

We evaluated cord formation in MB/BacT broth as a rapid method for presumptive identification of the Mycobacterium tuberculosis complex. Kinyoun acid-fast-stained smears from 370 positive MB/BacT bottles were examined for the presence of serpentine cording. The smears were examined independently by two observers. Observer 1 (the supervisor of the mycobacteriology laboratory) examined all of the smears while observer 2 (a clinical microbiologist not familiar with acid-fast bacillus [AFB] microscopy) examined 148 randomly chosen smears that were read by observer 1 without knowledge of which smear was which. The sensitivity, specificity, and positive and negative predictive values of cording for the presumptive identification of M. tuberculosis read by observer 1 were 88.2, 97.4, 99.2, and 69.7%, respectively. These values were reported at 90.6, 52.3, 82.8, and 69.7%, respectively, by observer 2. Our laboratory prevalence of M. tuberculosis among positive cultures was 78% during the time this study was conducted. At the time of positive signal of the MB/BacT bottles, the broth of the bottles had sufficient cell mass to allow for observation of the presence or absence of serpentine cording. The presence of cords in MB/BacT broth is a reliable criterion for rapid, predictive identification of theM. tuberculosis complex for laboratories with a high proportion of the M. tuberculosis complex when the smears are examined by a microbiologist who has experience with AFB staining.

Identification of mycobacteria species by molecular methods

“…As a result, it has been underlined that the observation of cord formation in diffusions by experienced microbiologists in laboratories where the growth rate of MTC is high, is rapid and reliable. 11 While cord formation and BACTEC NAP tests have been used as the golden standard until the beginning of the 1990s in the identification of mycobacteria types, it has left its place to molecular methods in recent years. 10 Although…”

Cord formation and colony morphology for the presumptive identification of Mycobacterium tuberculosis complex

“…Attorri et al (1), Mc Carter et al (6), and Yagupsky et al (9) reported improvements in the recognition of cord formation over time. Badak et al (2) observed that recognition of true cording can be difficult for inexperienced workers.…”

“…This result is similar to the findings of other reports. However, loose, incomplete pseudocords produced by some NTM may have been misinterpreted as true cording (2,6,9).…”

Comparative radiological features of disseminated disease due to Mycobacterium tuberculosis vs non-tuberculosismycobacteria among AIDS patients in Brazil

“…This test is highly specific for the identification of Mycobacterium tuberculosis complex (99%) [ 27 ]. In cases which NAP test could not be done, the identification of species were made by the visualization of the aspect of the colony – presence or absence of cord formation in liquid media – due to the high sensitivity and specificity of this method [ 28 , 29 ]. We did not use DNA probes for speciation of mycobacteria.…”