Pooling sputum samples to improve the feasibility of Xpert® MTB/RIF in systematic screening for tuberculosis

SETTING

Systematic screening for tuberculosis (TB) using Xpert® MTB/RIF.

OBJECTIVE

To determine whether pooling sputum samples for Xpert testing may improve the feasibility and cost-effectiveness of Xpert by reducing the number of Xpert tests required.

DESIGN

Mycobacterium tuberculosis-spiked sputum samples at low organism concentrations were used to mimic samples that are more likely to be found in the screening, compared to the diagnostic, setting. Using Xpert, pooled sputum samples were tested from a pooling ratio of 1 in 2 to 1 in 12.

RESULTS

A linear relationship between the pooling ratio and the Xpert MTB cycle threshold (Ct) value was found. As the sputum pooling ratio increased, the Ct value also increased. However, the slope of this increase was relatively small. In the majority of the samples pooled (75/96, 78.1%), Xpert was able to detect M. tuberculosis.

CONCLUSION

These findings suggest that sputum pooling may be a viable method of improving the feasibility and cost-effectiveness of large-scale sputum testing using Xpert in the TB screening setting.

The role of macroscopic sputum quality assessments to optimise sputum testing for tuberculosis

SETTING

Community-wide active case finding for tuberculosis (TB) using Xpert® MTB/RIF as the primary screening tool, Ca Mau Province, Viet Nam.

OBJECTIVES

To determine whether macroscopic sputum quality characteristics (sputum colour and volume) can be used to predict Xpert MTB-negative sputum and hence exclude sputum samples from testing.

DESIGN

Field staff conducted household visits to approximately 51,200 adults in 58 villages randomly selected from throughout the province. Sputum samples from all screened participants who were able to produce ⩾1 ml sputum underwent macroscopic sputum assessment and were tested with Xpert.

RESULTS

Of the 21,624 sputum samples tested, 159 (0.74%) were Xpert MTB-positive; 93% of the samples were 1-2 ml and nearly all were mucoid (93%) or mucopurulent (5.7%). One salivary sample was Xpert MTB-positive (2.0% of all salivary samples). The lowest positive predictive value for any sputum volume or colour characteristic was 0.66%. This was not substantially different from the overall prevalence of positive sputum Xpert MTB (0.74%).

CONCLUSION

Sputum colour and volume cannot be used to predict the presence or absence of M. tuberculosis in sputum detected using Xpert. These sputum quality parameters cannot therefore be used to exclude sputum samples from testing for TB.