Stronger spins boost TB microscope detection

Ziehl–Neelsen (ZN) smear microscopy is a mainstay of tuberculosis diagnosis and treatment monitoring worldwide, but it can miss infections because not all Mycobacterium tuberculosis are recovered during the pre-analytical steps. Godlove Chaula and colleagues set out to test whether simple changes to those pre-analytical steps — specifically the force and duration of centrifugation — could capture more bacteria and make the ZN smear more sensitive. To do this they ran controlled laboratory experiments with the laboratory strain M. tuberculosis H37Rv and with pulmonary TB sputum samples taken from people. All samples were first decontaminated using NALC–NaOH and then processed under different centrifugation conditions. The team compared three centrifugal forces and two spin times to see which combination recovered the most bacilli. They examined ZN smears in triplicate to grade the number of organisms and recorded whether each smear was positive or negative. Their analysis used standard statistical thresholds, with significance set at P < 0.05, to determine whether the changes made a reliable difference.

The methods were straightforward and focused on measurable differences. After NALC–NaOH decontamination, samples were centrifuged at 2,000, 3,000, or 6,000 × g for 40 min to test force, and the effect of centrifugation time at 3,000 × g was tested by comparing 20 and 40 min using the same specimens. ZN smear grading and positivity were evaluated in triplicate and compared statistically. In the controlled M. tuberculosis H37Rv suspensions, smear grading increased as centrifugal force rose, but smear positivity reached a plateau at 3,000 × g, indicating no further gain in the number of positive smears beyond that force. By contrast, clinical pulmonary sputum samples showed progressive increases in both smear grading and smear positivity as force increased, and the improvement in positivity was statistically significant (p = 0.0097). When the team extended centrifugation time at 3,000 × g from 20 to 40 min, smear positivity did not change for either laboratory suspensions or clinical sputum (P = 0.30).

Taken together, these results point to a practical way to reduce false-negative ZN smears without changing staining or microscopy techniques. For laboratory strains like M. tuberculosis H37Rv, the standard centrifugation conditions appear adequate, but for real-world clinical sputum the study found that using higher centrifugal forces improved bacillary recovery and increased the likelihood that a ZN smear would be positive. Because ZN smear microscopy is widely used in routine TB laboratories, optimizing the relative centrifugal force during pre-analytical processing could strengthen both diagnosis and treatment monitoring by making smears more sensitive. The work suggests that an adjustment in how samples are spun — rather than new reagents or equipment — may yield meaningful gains in detecting TB in sputum, potentially improving patient care in settings that rely on ZN smear microscopy.