Concentration of Mycobacterium tuberculosis from sputum using ligand-coated magnetic beads

Direct sputum smear microscopy is usually less sensitive than the indirect approach using concentration by centrifugation, but laboratories often do not have access to appropriate equipment. An alternative method of sample concentration has been developed based on magnetic beads coated with a polymeric ligand that binds mycobacteria species. The 'TB-Beads' technology allows manual sample preparation using simple magnetic separation equipment prior to microscopy.

1) To evaluate TB-Beads, in combination with fluorescent auramine staining, on a blind panel of 129 frozen samples from the Special Programme for Research & Training in Tropical Diseases (TDR), the World Health Organization sputum bank; and 2) to compare the microscopy results to indirect microscopy, culture and the clinical data already available on these samples.

The correlation between the TB-Beads protocol and indirect microscopy was 96.1% (124/129). The TB-Beads protocol was 89.4% (76/85) sensitive compared to culture and 77.8% (77/99) sensitive compared to clinical diagnosis.

Capture by magnetic particles yields a concentrated sample that is immobilised over a defined area of the slide, thereby aiding microscopic analysis. TB-Beads allow laboratories that currently perform comparatively insensitive direct microscopy to implement a concentration method that has the potential to improve TB detection rates.