Increased case finding of tuberculosis from sputum and sputum deposits after magnetic bead concentration of mycobacteria

• Magnetic bead concentration of mycobacteria was compared to direct smear on sputa.
• Magnetic bead concentration increases the scores of the same positive sputum samples.
• Magnetic bead concentration significantly increases the numbers of positive samples.
• Magnetic bead method is simple and compatible with use within a safety cabinet.

Concentration of mycobacteria from sputum by centrifugation prior to acid-fast microscopy increases case finding compared to direct microscopy of the sputum (direct smear). However, centrifugation has to be performed outside the safety cabinet and many laboratories do not have access to a centrifuge. Magnetic bead extraction of the mycobacteria is an alternative method that can be performed in a cabinet with just a magnet. Magnetic TB-Bead (Microsens Medtech Ltd) extraction of mycobacteria from sputum prior to microscopy was compared to direct smear on 78 sputum samples. Microscopy of the TB-Bead extracts identified all of 26 of the direct smear positive samples either with the same microscopy score or, in 19/27 of samples, with an increased microscopy score which aided microscopy detection. In addition, microscopy of the TB-Bead extracts identified 10 additional positive samples compared to direct smear; which represents a statistically significant increase in case finding of 38% (p = 0.002) compared to direct smear. In a separate study, TB-Beads enabled further 4 positive samples to be detected from 30 centrifuged pellets that were originally smear negative; two of these were subsequently found to be positive when the original deposits were reinvestigated by smear microscopy. By concentrating mycobacteria from sputum and sputum deposits, TB-Beads have been demonstrated to increase the number of positive sputum samples which could increase case-finding. The TB-Bead method is simple and rapid and compatible with use within a safety cabinet.