The Colour Test for drug susceptibility testing of Mycobacterium tuberculosis strains

Tartu, Estonia.

To assess the performance and feasibility of the introduction of the thin-layer agar MDR/XDR-TB Colour Test (Colour Test) as a non-commercial method of drug susceptibility testing (DST).

The Colour Test combines the thin-layer agar technique with a simple colour-coded quadrant format, selective medium to reduce contamination and colorimetric indication of bacterial growth to simplify interpretation. DST patterns for isoniazid (INH), rifampicin (RMP) and ciprofloxacin (CFX) were determined using the Colour Test for 201 archived Mycobacterium tuberculosis isolates. Susceptibilities were compared to blinded DST results obtained routinely using the BACTEC™ Mycobacteria Growth Indicator Tube™ (MGIT) 960 to assess performance characteristics.

In all, 98% of the isolates produced interpretable results. The average time to positivity was 13 days, and all results were interpretable. The Colour Test detected drug resistance with 98% sensitivity for INH, RMP and CFX and 99% for multidrug-resistant tuberculosis. Specificities were respectively 100% (95%CI 82-100), 88% (95%CI 69-97) and 91% (95%CI 83-96) and 90% (95%CI 74-98). Agreement between the Colour Test and BACTEC MGIT 960 were respectively 98%, 96%, 94% and 97%.

The Colour Test could be an economical, accurate and simple technique for testing tuberculosis strains for drug resistance. As it requires little specialist equipment, it may be particularly useful in resource-constrained settings with growing drug resistance rates.