Tuberculosis (TB)

Results: Detection and treatment of TB cases

Case notifications, 1995-2002

The 201 countries reporting to WHO in 2002 notified 4.0 million cases, of which 1.7 million (42%) were sputum smear-positive (Table 5, Annex 5). The global, crude notification rate (all forms of TB for all reporting countries) has been more or less stable since records began in 1980, and changed little between 2001 (62 per 100 000) and 2002 (66 per 100 000). By contrast, the total number of notified smear-positive cases increased by about 4% per year between 1995 and 2002, probably because of the emphasis placed by DOTS programmes on diagnosis by sputum smear microscopy. Based on notifications of all TB cases from countries thought to have reliable data, and where there has apparently been no significant change in case finding effort, we estimate that the global incidence rate of TB (all forms) was growing at 1.1% per year in 2002, and the total number of cases was growing at 2.4% per year.

The trends in case notifications between 1980 and 2002, and the presumed trends in incidence, differ among regions. The consistency in trend among countries within each region is revealed by the 95% CI on the standardized series of notification rates in Figure 4 . Although the notification rate of TB has been rising quickly in eastern Europe (5% per year, 1997--2002), and in African countries with high HIV prevalence (eastern and southern African countries; 7% per year), the rate of increase has been slowing in both regions since the mid 1990s (Figure 5). In most other regions of the world, the case notification rate has been roughly stable or in decline.

Figure 5: Annual changes in TB notification rates 1992–2002
Average percent change (on previous year) in notification rates (all forms, DOTS and non-DOTS) between consecutive years for 2 groups of countries; Africa – high HIV (red) and eastern European countries (grey). See Figure 4 for countries included.

This evaluation of trends in incidence has been used, with other data, to update estimates of TB incidence for every country and region of the world (Table 6 , Annex 5). There were an estimated 8.8 million (141 per 100 000) new TB cases in 2002, of which 3.9 million (63 per 100 000) were smear-positive. These revised incidence estimates are the denominators used to calculate case detection rates for 2002. The ranking of countries by number of TB cases has drawn attention to the 22 countries that account for roughly 80% of the world's burden of TB, but the importance of the TB problem for individual countries is better expressed as the incidence rate. Among the 15 countries with the highest estimated TB incidence rates per capita, 13 are in Africa and, in most, the prevalence of HIV infection among TB patients is high (Figure 6).

Figure 6
: Fifteen countries with the highest estimated TB incidence rates per capita (all ages, all forms; grey bars) and corresponding incidence rates of HIV-infected TB (among adults 15–49 years; red bars), 2002

Case notifications from African countries show two other patterns that appear to be associated with HIV infection. First, women aged 15--24 years make up a higher proportion of TB cases in countries with higher rates of HIV infection (Figure 7), consistent with the observation that HIV prevalence tends to be higher in women than men in this age range, and the difference between the sexes is bigger where HIV infection rates are higher. Second, some East African countries with high rates of HIV infection show a declining proportion of smear-positive cases among all TB cases notified (Figure 8). This is expected because smear-negative TB is more frequent among HIV-positive than HIV-negative TB cases, but might also reflect a decline in diagnostic performance, despite the emphasis placed on sputum smear microscopy in DOTS programmes.

Figure 7
: The proportion of notified TB patients aged 15–24 years that were women, plotted against the estimated HIV prevalence in adults 15–49 years. TB data are for 15 African countries in sub-Saharan Africa (2002); HIV estimates are from UNAIDS (2001); r2 = 66%.

Among all TB cases reported in 2002, 3.0 million (over two-thirds) originated in DOTS areas (Table 5). Of the smear-positive cases, 1.4 million were notified by DOTS programmes (83%). The African (25%), South-East Asia (37%), and Western Pacific Regions (20%) together accounted for 82% of all notified cases and similar proportions of smear-positive cases. Because DOTS emphasizes diagnosis by sputum smear microscopy, 47% of all new cases were smear-positive (45--60% expected) in DOTS areas, compared with 30% elsewhere. Similarly, 57% of new pulmonary cases were smear-positive under DOTS (55-70% expected), compared with 34% elsewhere.

Figure 8
: Smear-positive cases as a proportion of all notified cases over time for 6 African countries with high HIV prevalence

The increment in smear-positive cases detected by DOTS programmes was roughly constant between 1995 and 2000 (linear increase in total cases detected), but there are signs that case finding under DOTS has accelerated globally over the past 2 years. An extra 610 228 TB cases (all forms) were reported under DOTS between 2001 and 2002, as compared with the average of 269 268 over the period 1995--2000. Similarly, an extra 214 656 smear-positive cases were reported between 2001 and 2002, as compared with the 1995-2000 average of 134 157.

The number of cases enrolled under DOTS has continued to increase much more quickly than the total number of cases notified: DOTS programmes appear to have improved their performance primarily by recruiting cases that would otherwise have been notified outside DOTS programmes. Thus 25% more TB cases, and 18% more smear-positive cases, were recruited under DOTS in 2002 as compared with 2001. Conversely, the numbers of TB cases (both smear-positive and all forms) reported outside DOTS programmes fell by 28% between 2001 and 2002.

Approximately 28% of the additional smear-positive cases reported from all DOTS programmes in 2002 (compared with 2001) were in India. There were smaller but marked improvements in case detection in South Africa (contributing 12% of the total increase), Indonesia (10%), Pakistan (4%), Bangladesh (3%), and the Philippines (3%). These 6 countries together accounted for 61% of the additional cases notified under DOTS in 2002.

The global trade-off between cases recruited to DOTS programmes and at the same time lost from other programmes can also be seen in data from individual countries. Notifications from 51 countries show that the gain in DOTS areas is, by and large, offset by the loss from non-DOTS areas, and many of these countries cluster around the line of exact compensation (slope -1; Figure 9). India (not marked on the graph) gained 59 858 smear-positive cases under DOTS between 2001 and 2002, but notifications from outside DOTS programmes fell by 48 852, a net gain of 11 006 cases. Bangladesh, Brazil, Pakistan, and Zambia also made noticeable net gains (points lie above the line in Figure 9). China reported fewer cases from both inside and outside DOTS areas.


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