The role and performance of chest X-ray for the diagnosis of tuberculosis: A cost-effectiveness analysis in Nairobi, Kenya

Background: The objective of this study was to establish 1) the performance of chest X- ray ( CXR) in all suspects of tuberculosis ( TB), as well as smear- negative TB suspects and 2) to compare the cost- effectiveness of the routine diagnostic pathway using Ziehl- Neelsen ( ZN) sputum microscopy followed by CXR if case of negative sputum result ( ZN followed by CXR) with an alternative pathway using CXR as a screening tool ( CXR followed by ZN). Methods: From TB suspects attending a chest clinic in Nairobi, Kenya, three sputum specimens were examined for ZN and culture ( Lowenstein Jensen). Culture was used as gold standard. From each suspect a CXR was made using a four point scoring system: i: no pathology, ii: pathology not consistent for TB, iii: pathology consistent for TB and iv: pathology highly consistent for TB. The combined score i + ii was labeled as " no TB" and the combined score iii + iv was labeled as " TB". Films were re- read by a reference radiologist. HIV test was performed on those who consented. Laboratory and CXR costs were used to compare for cost- effectiveness. Results: Of the 1,389 suspects enrolled, for 998 ( 72%) data on smear, culture and CXR was complete. 714 films were re- read, showing a 89% agreement ( kappa value = 0.75 s. e. 0.037) for the combined scores " TB" or " no- TB". The sensitivity/ specificity of the CXR score " TB" among smear-negative suspects was 80%/ 67%. Using chest CXR as a screening tool in all suspects, sensitivity/ specificity of the score " any pathology" was 92%, respectively 63%. The cost per correctly diagnosed case was for the routine process $ 8.72, compared to $ 9.27 using CXR as screening tool. When costs of treatment were included, CXR followed by ZN became more cost- effective. Conclusion: The diagnostic pathway ZN followed by CXR was more cost- effective as compared to CXR followed by ZN. When cost of treatment was also considered CXR followed by ZN became more cost- effective. The low specificity of chest X- ray remains a subject of concern. Depending whether CXR was performed on all suspects or on smear- negative suspects only, 22% 45% of patients labeled as " TB" had a negative culture. The introduction of a well- defined scoring system, clinical conferences and a system of CXR quality control can contribute to improved diagnostic performance.