Interpretation of tuberculin reactions in populations with a high proportion of BCG-vaccinated persons.

In recent years there has been an apparent increase in the frequency with which inconsistent results are obtained in the routine tuberculin-testing carried out at the Central Tuberculosis Dispensary in Copenhagen. The author describes an investigation which was undertaken to find out the cause of these conflicting results. In this study, about a thousand non-tuberculous people, many of whom had previously been vaccinated with BCG, were tested by one of two different techniques. In another similar group of people, 300 tuberculin reactions were each read independently by three nurses.The pattern of tuberculin sensitivity observed among the nonvaccinated differed from that observed among the vaccinated. Whereas the reactions of the former could, with fair accuracy, be classified as "positive" or "negative" according to their size, those of the latter showed a unimodal distribution by size, so that any such classification was necessarily arbitrary. Moreover, it was found that many of the vaccinated individuals tended to give positive and negative reactions alternately to successive tests, owing to random variations in the testing techniques.The author concludes that it is impracticable to divide BCG-vaccinated people into "positives" and "negatives", and draws a parallel with populations in tropical countries, where the presence of "non-specific" sensitivity renders tuberculin-testing unreliable as a diagnostic tool.