Tuberculosis (TB)

Discussion

Detection and treatment of TB cases

Two hundred and one countries reported to WHO on the TB epidemic in 2002, more than in any previous year. The number of countries that had adopted the DOTS strategy increased to 180, and 69% of the world's population had access, in principle, to DOTS. Adding the 2002 case notifications to those of previous years, a total of 13.3 million TB patients, and 6.8 million smear- positive patients, were treated in DOTS programmes between 1995 and 2002.

The most critical markers of progress are case detection and treatment success rates. The smear- positive case detection rate increased to 37% globally, just over half way to the 70% target. Of 1.2 million smear-positive cases registered in the 2001 cohort, 82% were successfully treated, close to the 85% target, but no better than for the 2000 cohort. India reported the biggest gains in case detection among countries that provided data for both 2001 and 2002; the additional 59 858 smear-positive cases reported by the Indian DOTS programme represent 28% of the global improvement in case detection, in a country that has 20% of the world's case load. Other major increases in case detection were reported in South Africa, Indonesia, Pakistan, Bangladesh, and the Philippines.

Better case finding represents progress in TB control only when accompanied by high cure rates. Of the countries that have been most progressive on case detection, South Africa still reports a very low rate of treatment success (65%). If low treatment success means frequent treatment failure in this country, then drug resistance will be the outcome: the 2001-2 survey of resistance across South African provinces found MDR-TB prevalence rates of up to 14% among previously treated patients.6

The six countries listed above were together responsible for over 60% of the increase in cases detected, and mostly responsible for the acceleration in case finding. An additional 214 656 cases were reported during 2002, as compared with 2001, which is 60% greater than the average increase between 1995 and 2000. The step-up in recruitment to DOTS programmes is even more pronounced in the numbers of all TB cases (smear-positive and smear-negative) reported. However, even with this acceleration, the 2002 data show that the world's TB control programmes are not yet on course, collectively, to meet the 70% target by 2005. That would require an annual increase of about 433 000 smear-positive cases in each of the years 2003--5.

Among the HBCs, only Viet Nam has reached both targets, though Cambodia, Myanmar and the Philippines appear to be close. By the end of 2002, 63 countries lay in the penumbra of the target zone (case detection > 50%, treatment success > 70%), but together accounted for only 15% of the smear-positive case load globally.

Some gains in case detection (as defined by WHO) could be made rapidly in countries and regions where many cases are already known to public health authorities (assuming they are really TB cases), but are not treated under DOTS. Data from the Americas and Europe indicate that the target for case detection could be met, or closely approached, just by ensuring that the diagnosis and treatment of known TB patients meets DOTS standards. Significant gains in case detection could be made in South-East Asia for the same reason. Although there is little scope for making similar gains in Africa, the Eastern Mediterranean and Western Pacific regions (where most patients are already reported under DOTS), the combined total of all such patients would push global case detection from 37% up to around 50%, the same as the fraction of all TB cases found in 2002.

To go beyond 50% case detection will be challenging, if the pattern of DOTS expansion observed from 1995 to 2002 persists. The data in this report identify two obstacles en route to the 70% target. The first is the relatively sluggish increase in case notifications from all sources (DOTS and non-DOTS). The number of smear-positive cases notified to WHO by public health authorities increased by just 4% per year between 1996 and 2002, and the total number of TB cases has not increased at all. Consequently, the proportion of all notified smear-positive cases that come from DOTS programmes has been increasing since 1995. If this trend continues, all TB cases reported to WHO in 2005 will be notified and treated by DOTS programmes. This means that all TB patients reported in the public sector will, by 2005, receive the internationally recommended standard of care. But it also means that, to reach the 70% target by 2005, DOTS programmes must recruit cases that would not otherwise have been notified in the public sector. The rate of recruitment of TB cases to health programmes that participate in the public case notification system has hitherto been slow.

The second impediment is that the smear-positive case detection rate within DOTS areas, as measured by the ratio of case detection to population coverage, has remained roughly constant since 1996, averaging 49%. That is, almost all of the gains in case detection made under DOTS have been made through geographical expansion, and not by improving case finding in established DOTS areas. If this continues to be true, the smear-positive case detection rate will still be roughly 50% even when, according to measures of population coverage, the whole world has access to DOTS. Some HBCs do show improvements in case finding within DOTS areas, especially India, Indonesia, Bangladesh, and the Philippines, but these are much slower than the improvements made by extending DOTS to new areas.

Among the 1.2 million smear-positive cases treated under DOTS in the 2001 cohort, 82% were reported to have successful outcomes. HIV co-infection is blamed for relatively poor results in Africa (71%), and HIV may indeed contribute to the high death rate (7%). However, African NTPs could do substantially better by cutting the proportion of patients lost from DOTS cohorts, which amounted to 21% of patients in 2001. In eastern Europe, relatively high rates of drug resistance could help to explain why 12% of patients failed treatment and 7% died. But these data need closer examination: it is possible that a proportion of the "failures" had not completed treatment after 6 months because, for example, longer regimens are used to treat patients with resistant bacilli. For these patients, the final outcome of treatment is not known.

In summary, the global, smear-positive case detection rate was 37% in 2002, over half way to the 70% target, and rising more quickly than at any time since 1995. Given recent trends, we expect the smear-positive case detection rate by DOTS programmes to be about 50% in 2005, by which time all TB patients notified and treated in the public sector will receive the internationally recommended standard of care. Case detection could be increased from 37% to 50% by ensuring that the diagnosis and treatment of known TB cases in the Americas, South-East Asia, and the Western Pacific Regions conforms with DOTS standards. To get above 50% case detection will be demanding because the notification rate of all TB cases by public health authorities has been stable at about this level for many years, and because DOTS programmes will probably have exhausted this supply of cases by 2005.

Two years ago, we forecast that the smear-positive case detection rate would accelerate after year 2000, and then saturate below 50% around 2005.24 The latest data suggest a somewhat brighter future, but remain consistent with the notion that saturation will follow acceleration. To escape that future, DOTS programmes and public health authorities must now do something different. They must recruit patients from non- participating clinics and hospitals, notably in the private sector in Asia, and from beyond the present limits of public health systems in Africa. These are the regions of the world that account for the vast majority of cases that are not seen, and therefore not yet "detected", by public health authorities.

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