Still screening for pulmonary tuberculosis?

The introduction of mass radiography was an important event for the detection of pulmonary tuberculosis after the Second World War, when the prevalence of disease was high and the accessibility of health services for the public was low. The detection rate was considerable and the method contributed the antituberculosis programmes a great deal; the quesl.ion of controlled design and follow-up of screening procedures was of no interest at that time. Mass miniature radiography for pulmonary tuberculosis was still, in the 1970's, the largest screening procedure ever carried out on the initiative of the public health services. The WHO expert Committee on Tuberculosis [1) shook the system by recommending the phasing out of indiscriminate case finding by mobile mass radiography. By this time many countries were beginning to nol.ice the lack of inbuilt evaluation and of calculations of the costs and benefits of the system; the time was gradually becoming ripe for the reassessment of the programmes. The idea behind periodic screening for tuberculosis was to detect all the sources of infection at an early stage and thus to achieve a favourable epidemiological impact. Even before the advent of chemotherapy, it was known that the most "dangerous" patients are those with cavities and with enormous amounts of tubercle bacilli in their sputum, i.e. smear-positive cases. By contrast, patients positive by culture infect their contacts only slightly more than smearand culture-negative patients. Consequently, it is not justified to classify smear-positive and culture-positive patients together in terms of epidemiology. This fact still remains valid in low prevalence countries. Only in a few mass radiography surveys have the detected cases been grouped according to the bacteriological status; in most surveys all the "tuberculosis cases" discovered are listed together. In research conditions, however, the yield of smear-positive patients in mass radiography has not been more than about onefourth [2]. In the Finnish programme evaluated in 1984 only 11 per cent of smear-positive cases and 18 per cent of all cases of tuberculosis were detected by selective mass screening (Liippo, unpublished). Certain countries also include not only cases discovered by screening but also those diagnosed at out-patient clin-