Decontamination methods for samples preserved in cetylpyridinium chloride and cultured on thin-layer agar

SETTING: Long transportation times of samples to culture laboratories can lead to higher contamination rates and significant loss of viability, resulting in lower culture positivity rates. Thin-layer agar (TLA) is a sensitive culture method for the isolation of Mycobacterium tuberculosis that has been optimised with N-acetyl-L-cysteine-sodium hydroxide (NALC-NaOH) decontaminated samples. The combination of the TLA culture method and other decontamination procedures has not been extensively validated.DESIGN: Among 390 smear-positive samples, we compared the culture positivity of samples decontaminated using the Petroff method vs. NALC-NaOH neutralised with phosphate buffer (PBS), applied to samples preserved with cetylpyridinium chloride (CPC) or CPC-free, and then of CPC-preserved samples decontaminated with NALC-NaOH neutralised using Difco neutralising buffer. The sediments were inoculated on TLA, and then on MGIT 960 or Löwenstein-Jensen (LJ) as gold standards.RESULTS: Decontamination with NALC-NaOH yielded higher culture positivity in TLA than in the Petroff method, which was further enhanced by neutralising CPC with the Difco buffer. Surprisingly, culture positivity on LJ also increased after using Difco buffer, suggesting that CPC may not be completely neutralised in egg-based medium.CONCLUSIONS: After transportation in CPC, decontamination using NALC-NaOH followed by neutralisation using Difco buffer resulted in the best recovery rates for samples inoculated on TLA and on LJ.