Methods: Monitoring the detection and treatment of TB cases
The term "detection", as used in this report, means that a patient is diagnosed as having TB (correctly or incorrectly), and is reported within the national surveillance system, and then to WHO. Smear-positive cases are the focus of DOTS programmes because they are the principal sources of infection to others, because sputum smear microscopy is a highly-specific (if somewhat insensitive) method of diagnosis, and because patients with smear-positive disease typically suffer higher rates of morbidity and mortality than smear-negative patients.
We calculate the proportion of new sputum smear-positive cases out of all new pulmonary cases, which has an expected value of 65--80% in areas with a low prevalence of HIV infection.10 We calculate the case detection rate by dividing the number of notified smear-positive cases by the number of new cases estimated for that year. Detection is presented in two ways -- as the case detection rate (countrywide) and as the DOTS detection rate (by DOTS programmes):
The case detection rate (CDR) and the DOTS detection rate (DDR) are identical when a country reports only from DOTS areas. This should happen only when DOTS coverage (see below) is 100%.
Although these indices are termed "rates", they are actually ratios. The number of cases notified is usually smaller than estimated incidence because of incomplete coverage by health services, under-diagnosis, or deficient recording and reporting. However, the calculated detection rate can exceed 100% if case finding has been intense in an area that has a backlog of chronic cases, if there has been over-reporting (e.g. double-counting) or over-diagnosis, or if estimates of incidence are too low.
10 WHO. Tuberculosis Handbook. Geneva, WHO/TB/98.253.