Iterative evaluation of mobile computer-assisted digital chest x-ray screening for TB improves efficiency, yield, and outcomes in Nigeria

Description

Notes

Methods

These are data from digital LP chest x-ray images read by CAD4TB version 5 merged with data from 2 G4 Genexpert machines, with a data variable to indicate screening date, cough status, as well as age and sex. 

Two large container trucks housing a lead-lined chest-x-ray suite, reading area, and mobile GeneXpert laboratory were sourced via competitive tendering. Solar panels, shade canopies, anti-theft features were added.

CAD4TB version 4 by Delft Imaging interprets a digital image in less than a minute and can do so consistently during day-long screening events.24–26 The software generates a TB likelihood score between 0 and 100, indicating the extent of lung abnormalities. CAD4TB displays a heatmap pinpointing the size and location affected. The scores are used to set a threshold above which persons are invited for TB testing. Each team had a radiology technologist, laboratory technologists, data clerks, truck driver, and a clinical supervisor. Training was conducted on standard operating procedures (SOPs) drawn from procedural manuals of successful digital chest X-ray (CXR) TB screening programs.27 Experts in social mobilization crafted logos and messaging. The intervention was implemented by KNCV TB Foundation (KNCV) under a cooperative agreement, Challenge TB Project, funded by the United States Agency for International Development (USAID) from 2016 to 2019. 

The community mobilization strategy was geared toward recruitment of men and persons over 30 years of age, because of their elevated vulnerability. Men tend to be less likely to participate in community TB screening, so dedicated efforts are often required to attain a high risk participant mix.20,28 Aiming for a high-risk pool with a pulmonary TB prevalence of 1,000/100,000 per population with an estimated 85% sensitivity of the algorithm, we expected an average daily yield of 1.7 persons with bacteriologically-confirmed TB.

The intervention was carried out in three phases across four states (See Figure 2 of Eneogu et al. in PLOSGH).  First, a "Calibration phase" was undertaken to identify a feasible CAD4TB score threshold for GeneXpert test eligibility that would ensure a reasonable TB yield given varying micro-epidemic conditions and to ensure value for money.. Rigorous calibration would have required TB testing of approximately 30,000 people at low risk over a six-month period, at cartridge cost of 300,000 USD.  Instead, a sensitive algorithm comprised of a low testing threshold (CAD4TB score ≥ 40) and a classic symptom screen (cough of ≥two weeks) were trialed over 8 days (n=1875).  Persons with CAD4TB scores ≥ 80 and negative spot TB test results were followed up three to six months later to identify missed TB from GeneXpert MTB/RIF testing on a single spot sample. Emphasis was placed on implementation of a simple algorithm to facilitate the highest volume screening of highest risk adults while minimizing the participation burden and risk/benefit balance. Pilots were executed in two regions (North, South), to field test the approach. The third phase ("Scale-up") leveraged learning from the calibration and pilots to refine strategy. Persons classified as presumptive for TB followed the national guidelines.  Before treatment initiation, a risk factor and clinical interview of bacteriologically confirmed PTB was added to preclude over-diagnosis.Findings from each phase informed the design of the subsequent phase.  Iterative modification of procedures, strategies, CAD test thresholds and targets occurred after review of prior phase results.