WHO report 2008
Global tuberculosis control
1.5 Case detection rates
1.5.1 Case detection rate, all sources (DOTS and non-DOTS programmes)
The 2.5 million new smear-positive cases notified in 2006 from all sources (i.e. DOTS and non-DOTS programmes) represent 62% of the 4.1 million estimated cases (Table 1.2, Table 1.3; Annex 3). This is a small increase from a figure of 60% in 2005, following a slow and linear increase from 35% to 43% between 1995 and 2001 and a more rapid increase from 43% to 60% between 2001 and 2005 (Figure 1.10a). The improvement that occurred between 2002 and 2006 was attributable mostly to increases in the numbers of new smear-positive cases reported in the Eastern Mediterranean, South-East Asia and Western Pacific regions (Table 1.4).
The Region of the Americas and the European Region reported the largest numbers of new smear-positive cases from outside DOTS programmes. Counting all smear-positive cases from all sources, the case detection rate in the Region of the Americas was 76% (Table 1.4, Figure 1.11a). Counting all new cases (pulmonary and extrapulmonary) from all sources, the overall case detection rate in Europe was 70% (Figure 1.11b).
The 5.1 million new TB cases (all forms) that were notified from all sources in 2006 represent 56% of the 9.2 million estimated new cases. This is a further improvement from 2005, and continues the upward trend that began in 2002, following several years in which the detection rate had remained stable at 40–50% (Figure 1.10b).
1.5.2 Case detection rate, DOTS programmes
The principal WHO measure of case detection is the rate of case detection for new smear-positive cases in DOTS programmes, i.e. the number of new smear-positive cases detected by DOTS programmes divided by the estimated number of incident smear-positive cases. In 2006, DOTS programmes detected 2 496 478 new smear-positive cases (99% of all new smear-positive cases that were notified) out of an estimated 4.1 million new smear-positive cases, giving a case detection rate of 61% (Table 1.4, Figure 1.10a). The point estimate of a 61% case detection rate for 2006 is still below the 70% target set for 2005. There is, however, much uncertainty surrounding this estimate: the calculated 95% confidence limits range from 55% to 75%, but this does not account for all sources of random and systematic error.
New smear-positive case detection rates by DOTS programmes in 2006 were lowest in the African (46%) and European (52%) regions and highest in the Western Pacific Region (77%), the South-East Asia Region (67%) and the Region of the Americas (69%; Table 1.4, Figure 1.11, Figure 1.12). The Western Pacific is still the only region to have exceeded the 70% target, although the Americas (69%) and the South-East Asia regions (67%) fall just short on 2006 estimates. The particularly low figure for Europe compared with the overall case detection rate for all forms of TB of 70% (Figure 1.11b) suggests two major reasons for failing to reach the WHO target in this region: incomplete geographical coverage of DOTS and lack of emphasis on sputum smear microscopy (countries in the European Region report substantial numbers of cases in whom disease is diagnosed by methods other than sputum smear microscopy, and these cases are not necessarily smear-negative). In the Region of the Americas, the target of a 70% case detection rate for new smear-positive cases in DOTS programmes could be achieved simply by expanding the geographical coverage of DOTS programmes.
Although case detection of new smear-positive cases improved globally between 2005 and 2006, the rate of increase slowed compared with previous years: the increment between 2005 (58%) and 2006 (61%) was just 3%, the smallest reported annual increase since 1999–2000 (Table 1.14, Figure 1.10a). In the South-East Asia Region, the acceleration in case-finding after 2000 was attributable mostly to progress in Bangladesh, India, Indonesia and Myanmar. The more recent deceleration in detection is mainly a result of slowing DOTS expansion into India’s northern states, as the Indian national TB control programme (NTP) reaches full national coverage. The Western Pacific Region is dominated by China, where case-finding expanded rapidly between 2002 and 2005. However, China has made no progress in case-finding since reporting that the 70% target had been met in 2005 (Table 1.3, Table 1.4; Annex 1). The South-East Asia and Western Pacific regions are now slowing global progress in case detection.
DOTS programmes detected 4 990 374 new cases in 2006 (98% of all notifications) out of a total of 9.2 million estimated cases (Table 1.2, Table 1.3). This is equivalent to a case detection rate (all new cases) of 54%.
1.5.3 Case detection rate within DOTS areas
The case detection rate within DOTS areas (measured by the ratio of case detection to DOTS population coverage) changed little between 1995 and 2001, averaging 50% worldwide. Subsequently, it has increased to 66% in 2006 (Figure 1.13). This illustrates how increases in case detection rates in DOTS areas have made an important contribution to the overall improvement in case detection since 2001.
1.5.4 Number of countries reaching the 70% case detection target
National estimates of the case detection rate suggest that 77 countries met the 70% target by the end of 2006. Of the additional new smear-positive cases reported by DOTS programmes in 2006 (compared with 2005), 30% were in India and 33% were in Bangladesh, Pakistan and Indonesia (Figure 1.14).
While China and India have made big improvements in case detection in recent years, these two countries still accounted for an estimated 28% of all undetected new smear-positive cases in 2006. In 2006, as in 2005, Nigeria succeeded China as the second largest reservoir of undetected cases (10%). These three countries are among eight that together accounted for 59% of all smear-positive cases not detected by DOTS programmes in 2006 (Figure 1.15).
1.5.5 Prospects for future progress
It is inevitable that progress in case-finding of new smear-positive cases will slow as HBCs reach nationwide DOTS coverage, but the rate of increase in case detection is decelerating before reaching the 70% target globally. To compensate for slower progress in the regions where case detection is above (Western Pacific) or close to (South-East Asia) the target, faster progress is needed where case detection is lower, namely in the African (46%), the Eastern Mediterranean (52%) and European (52%) regions. The African Region is the most important in absolute terms; based on the latest estimates, it accounts for 75% of the “missing” cases among these three regions, with Ethiopia and Nigeria alone accounting for more than one-quarter of missing cases in these three regions.
The implication that DOTS programmes in the African Region in particular need to improve case detection comes with an important caveat. Efforts to assess improvements in case detection in the African Region have been confounded by the upward trend in incidence linked to the spread of HIV infection, such that it has been difficult to disentangle the effect of better programme performance leading to better case-finding, and the impact of the HIV epidemic, on increases in case notifications. In this context, a detailed investigation of DOTS implementation in Kenya found that the rise in smear-positive notifications from 92 to 107 per 100 000 between 2000 and 2006 was mostly due to an increase in case detection, rather than an increase in TB incidence linked to HIV. Consequently, the case detection rate has increased to 70% in 2006 (see also Annex 1).1 Similar investigations in other African countries may reveal that case detection is higher than stated in this report, and perhaps increasing more quickly than portrayed in Table 1.4.
1 Mansoer J et al. Estimating changes in the tuberculosis case detection rate in Kenya (submitted for publication).