There is a growing interest in the use of chest x-rays (CXR) as a screening tool for TB within enhanced case finding programs . TB prevalence surveys show that CXR has higher sensitivity than verbal screening for identifying pulmonary TB . Previously, costs, limited access to x-ray facilities, maintenance of equipment, availability of trained personnel and inter-observer variation meant that the role of CXR within diagnostic algorithms was limited .
CXR is widely used for detection of TB for a long time in especially North America and Europe, but many factors until recently complicated its use in low and medium resource countries. Digital radiology allows for low running costs, high image quality, lower radiation dose and automated reading (CAD) eliminates inter-observer variation. dCXR can be used as a low cost and rapid triage test to select individuals for further sputum examination and it allows for high throughput of large groups of people. dCXR has a high negative predictive value for pulmonary TB , is rapid (1 minute) and costs less than 25% of one Xpert® test, depending on settings.
Digital X-ray will give patients access to fast, accurate and convenient TB screening, which can contribute to reducing their travel cost, waiting time and speeding up access to diagnosis and treatment. Intensified TB case finding using digital radiology will support the combat against TB in people living with HIV who often present with smear negative sputum. New technology X-ray allows for low dose exposure for patients, making it an appropriate tool for screening of key populations. In combination with molecular tests, the time to diagnose active TB can be shortened from often 10-20 days to several hours. Digital radiology can also be used to spot pre-clinical TB and can support the health system in general, through diagnostic imaging of also other body parts.
High image quality
Digital X-rays are instantly available on the diagnostic monitor with constant high quality. Software tools to zoom in or to add contrast to the digital image enhance ease and accuracy of TB screening. Issues of poor readability and deterioration or loss of films are eliminated.
Low image cost
Films and developer are no longer needed, storage of images is done electronically and superior readability allow for a very low cost per image. Cost savings relative to analog systems using films in high throughput settings can exceed € 50.000 per year per system. Even at a low throughput of 30 images per day, digital images are more economic.
Instant image retrieval
Digital radiology systems using an integrated Picture Archiving and Communications System (PACS) can efficiently archive and retrieve large numbers of digital X-rays. This eliminates the high archiving cost of analog film based X-ray images. Using the universal Digital Imaging and Communications in Medicine (DICOM) format, the PACS attaches patient information to the right X-ray , eliminating the risk of “storing a film in the wrong envelope” and, enhancing patient safety.
Digital X-ray is clean technology as chemical waste from film developing, that can cause serious environmental damage, is completely eliminated.
Remote diagnosis - tele-radiology
If the X-ray image cannot be interpreted locally because the specific skills are not available or the image is particularly difficult to interpret, it can be sent over the internet or even over a low bandwidth (mobile phone) connection to a centre where expert reading can be done. The diagnosis can be sent back immediately and continues to be attached to the right digital Chest X-ray.
Computer Aided Detection
CAD allows for the automated recognition and quantification of abnormalities consistent with TB in a digital Chest X-rays (please see CAD description). Depending on the budget NTPs can select an abnormality threshold for Xpert® eligibility which allows for opting for higher sensitivity at reduced specificity (lower CAD score threshold) and vice versa (higher CAD score threshold). Zambart in Zambia put this threshold at > 60% CAD abnormality score resulting in a high predictive value (23%) for a positive Xpert® MTB/RIF test .
At recent UNION World Conferences the high sensitivity of Chest X-ray was illustrated by findings in the TB prevalence surveys in Namibia, Viet Nam, Ethiopia, Kenya, Malawi, Rwanda, Ghana and Cambodia. IRD presented at the 2018 UNION in The Hague the Zero TB initiative (cost) effectiveness using digital radiology as a rapid triage before Xpert®, compared to screening on symptoms only (please see below). The fact that screening on symptoms suggestive of TB only can result in missing more than 50% of TB cases, now makes Chest X-ray a mandatory part of any TB prevalence survey.