Trade-offs between cost and accuracy in active case finding for tuberculosis: A dynamic modelling analysis
Fig 1
Schematic illustration of the model.
The tuberculosis (TB) transmission model distinguishes TB by smear status and by symptom status. Upon developing symptoms, symptomatic individuals seek care through either the private or public sectors (‘passive’ TB services) after a certain delay, estimated to match data in Table 2. Although not shown here for clarity, the model captures these sectors separately, including the lower standard of TB care in the private sector (see S1 Text for full model details). Those successfully diagnosed initiate TB treatment; we assume that 15% of diagnoses in the public sector are conducted with Xpert, the remainder by microscopy. All those lost from the TB care cascade, whether because of missed diagnosis, pre-treatment loss to follow-up, or failed treatment, temporarily disengage from care-seeking, before once again seeking care after a given delay. Compartments shown in orange denote the effect of an active case finding (ACF) intervention on this ‘passive’ system; we assume that ACF consists of initial symptom screening, followed by microbiological confirmation. Meanwhile, the separate compartments on the right represent a subset of the general population that may be detected by the ACF intervention (represented by the orange compartments) because they have TB-like symptoms, but without TB: These may include, for example, individuals with chronic obstructive pulmonary disease, bronchitis, and other lung conditions. They incur a cost to the health system for diagnosis and—if they are mistakenly diagnosed with TB—a cost in false-positive treatment. The number incorrectly identified as having TB is dependent on the specificity of both the screening and confirmatory stages. Finally, at any stage individuals may die of natural causes or of TB (in the diseased compartments) or recover spontaneously. For simplicity, these transitions are not shown in the figure (see S1 Text for full model details).