gtbr2024

1.4 National TB prevalence surveys

Rationale: why and where are surveys relevant?

To reliably track the burden of tuberculosis (TB) disease in terms of TB incidence and TB mortality from subnational to global levels, the ultimate goal is that all countries can rely on data routinely collected through a) national disease surveillance systems and b) vital registration (VR) systems in which causes of death are coded according to the international classification of diseases (ICD).

Currently, all countries have national systems for notification (i.e. reporting) of people diagnosed with TB and almost all countries report TB case notification data to the World Health Organization (WHO) on an annual basis (Section 2.1). However, in many countries (including most high TB burden countries) the number of notified cases each year is not a good proxy for the actual number of people who develop TB disease, for two reasons. The first is underreporting of people diagnosed with TB, especially in countries with large private sectors or in which people with TB seek care in public facilities that are not linked to the national TB programme and its associated reporting systems. The second is underdiagnosis, especially in countries with geographic or financial barriers to seeking and accessing health care. Many countries (including most high TB burden countries) do not have established national VR systems of high quality and coverage that can be used to reliably monitor the number of deaths and their cause (1).

In countries with a relatively high burden of TB disease that do not yet have national disease notification systems or national VR systems with cause-of-death data that are of sufficiently high quality and coverage, national TB prevalence surveys are the best way to directly measure the burden of TB disease in the population (2–5). In terms of disease burden, WHO currently recommends consideration of surveys according to previously documented epidemiological criteria (2, 3, 5). These criteria, and the countries that currently meet them, are shown in Box 1.4.1.

 

Box 1.4.1

Criteria for assessment of whether a national TB prevalence survey is relevant

A. Countries that have conducted at least one survey since 2007

  1. Prevalence of bacteriologically confirmed pulmonary TB ≥250 per 100 000 population aged ≥15 years estimated by the most recent survey. This ensures that the sample size will be <70 000 individuals, such that a survey is feasible in terms of cost and logistics.

  2. At least 10 years have elapsed since the most recent survey. This ensures that enough time has passed to allow measurement of a statistically significant change in prevalence between the two surveys.

The 16 countries that currently meet both criteria are Bangladesh, the Democratic People’s Republic of Korea, Ethiopia, Ghana, Indonesia, Kenya, the Lao People’s Democratic Republic, Malawi, Mongolia, Nigeria, Pakistan, the Philippines, Uganda, the United Republic of Tanzania, Zambia and Zimbabwe.

A further 9 countries will meet both criteria during the period 2027—2030: Eswatini, India, Lesotho, Mozambique, Myanmar, Namibia, Nepal, South Africa and Viet Nam.

B. Countries that have never conducted a survey

  1. Estimated TB incidence ≥150 per 100 000 population per year (all forms, all ages). This ensures that the sample size will be <70 000 individuals, such that a survey is feasible in terms of cost and logistics.

  2. No national or sample vital registration (VR) system of high coverage and quality is available. VR systems need to include coding of causes of deaths according to international standards. Without such a system there is no reliable direct measurement of TB disease burden.

  3. Universal health coverage (UHC) service coverage index score is <80 (SDG Indicator 3.8.1) (6). This is an indirect indicator of insufficient access to quality health services, as defined in the WHO TB surveillance checklist of standards and benchmarks (second edition) (7).

The 25 countries that currently meet all three criteria are Afghanistan, Angola, Bhutan, Botswana, Cameroon, the Central African Republic, Chad, the Congo, Côte d’Ivoire, the Democratic Republic of the Congo, Djibouti, Equatorial Guinea, Gabon, Guinea, Guinea-Bissau, Haiti, Kiribati, Liberia, Madagascar, Marshall Islands, Micronesia, Papua New Guinea, Sierra Leone, Somalia and Tuvalu.

What is measured in a survey?

National TB prevalence surveys can provide a reliable measurement of the number of people in the population with bacteriologically confirmed pulmonary TB at a given point in time, and the distribution of these cases by age and sex. In addition, repeat surveys allow assessment of trends, and of the impact of interventions to reduce the burden of disease in the period since the last survey. WHO recommends that surveys focus on people aged 15 years or over (2, 3).

How can survey results be used?

Results can be used to inform national estimates of TB incidence in all age groups, and can thus help to track progress towards the milestones and targets for reductions in TB incidence set in the WHO End TB Strategy (Section 1.1). Previously, survey results were also important for the assessment of progress towards global, regional and national targets for reductions in TB prevalence between 1990 and 2015.

For these reasons, the implementation of national TB prevalence surveys in 22 priority countries (referred to as global focus countries, GFCs) was one of three strategic areas of work defined by the WHO Global Task Force on TB Impact Measurement (the Task Force) for the period 2007–2015 (2, 4). National TB prevalence surveys were retained within the Task Force’s updated strategic areas of work after 2015 (5, 8).

Other benefits of prevalence surveys include:

  • They provide reliable evidence about the distribution of the burden of TB disease by age and sex; this may be different from the distribution suggested by case notification data.

  • They allow assessment of how case detection gaps vary by age and sex. This can be done by comparing the ratio of prevalence to notifications by age group and sex. Such evidence can potentially inform the development and implementation of policies or interventions to narrow these gaps.

  • They provide information about the symptomatic status of people with TB disease in the community, since all survey participants are initially screened using both a chest X-ray and an interview about symptoms. Prevalence survey data have been the key source of data recently used to highlight and discuss the implications of “asymptomatic TB” (see also the featured topic on asymptomatic TB).

  • Surveys can be used to collect data about the health care seeking behaviour of people with TB disease, and in turn assessment of what improvements to health services may be required to ensure more prompt TB diagnosis and treatment.

  • Repeat surveys can be used to assess trends in TB disease burden and to evaluate the impact of interventions implemented since the last survey.

  • If data for people identified to be on TB treatment at the time of the survey are compared with official notification data, the level of underreporting of people diagnosed with TB to the national surveillance system can be assessed. Results can be used to inform the design and implementation of corrective actions, if required.

  • Survey findings can be used to advocate for action needed to improve TB prevention, diagnosis and treatment, and associated resource mobilization.

Status of progress

National TB prevalence surveys became a strategic area of work for the WHO Global Task Force on TB Impact Measurement in 2007. Between 2007 and August 2024, a total of 36 national TB prevalence surveys in 32 countries were implemented using the screening and diagnostic methods recommended by WHO (Fig. 1.4.1). These 32 countries comprised 17 in Africa and 15 in Asia, and 20 of the 22 GFCs. During this period, five countries implemented repeat surveys: China, Cambodia, Myanmar, the Philippines and Viet Nam. Timor-Leste completed its first survey in December 2023, and Cambodia completed its third survey in May 2024 (Fig. 1.4.2). These numbers represent a major surge in the number of surveys implemented worldwide, following two decades in which only a few surveys were completed, mostly in the WHO Western Pacific Region (4).

Countries that are actively interested in implementing a repeat prevalence survey before 2030 include Ethiopia, Ghana, Malawi, Nigeria, Uganda, the United Republic of Tanzania, Zambia and Zimbabwe in Africa; and Bangladesh, Indonesia, Pakistan and Thailand in Asia. Botswana is interested in implementing a first survey.

Fig. 1.4.1 National surveys of the prevalence of TB disease, 2007–2024

The year in which most field operations were implemented is shown. African countries are shown in purple, and Asian countries in green.
imeline of national TB prevalence surveys implemented since 2000
a The survey in Bangladesh (2008) collected sputum samples from all individuals (aged ≥15 years), and did not use chest X-ray and/or a symptom questionnaire to screen individuals for sputum submission.


Map of countries involved in national TB prevalence surveys
a A survey was conducted in accordance with WHO recommendations included in Tuberculosis prevalence surveys: a handbook (2011) and a report has been published. Screening included a chest X-ray and an interview about symptoms; at least culture was used to confirm diagnosis. The most recent surveys in Bangladesh, Cambodia, Eswatini, India, Kenya, Lesotho, Myanmar, Mozambique, Namibia, Nepal, the Philippines, South Africa, Timor-Leste and Viet Nam used both culture and Xpert assays for diagnosis

b Repeat survey under consideration: a country has developed a protocol (e.g. Pakistan, Uganda) or expressed strong interest in conducting a repeat TB prevalence survey.

Survey findings and implications

Surveys have shown that the estimated prevalence of bacteriologically confirmed pulmonary TB per 100 000 population aged 15 years or over was high in many countries, but there was also considerable variation (Fig. 1.4.3).

Fig. 1.4.3 Estimates of the prevalence of bacteriologically confirmed pulmonary TB (≥15 years of age) in surveys completed 2007–2021a

a The measured prevalence of bacteriologically confirmed pulmonary TB was higher in the 2017 survey in Viet Nam compared with 2007. However, this was due to more diagnostic testing with more sensitive methods. When results based on the same method were compared, prevalence was estimated to have fallen between 2007 and 2017. Results from recently completed surveys in Cambodia and Timor-Leste are pending.

In African countries, prevalence ranged from 119 (95% confidence interval [CI]: 79–160) per 100 000 population in Rwanda (in 2012) to 852 (95% CI: 679–1026) per 100 000 population in South Africa (in 2017). In Asian countries, prevalence ranged from 119 (95% CI: 103–135) per 100 000 population in China (in 2010) to 1159 (95% CI: 1016–1301) per 100 000 population in the Philippines (in 2016).

In most Asian countries and some African countries, prevalence increased with age (Fig. 1.4.4, Fig. 1.4.5).

Fig. 1.4.4 Estimated age-specific prevalence of bacteriologically confirmed pulmonary TB for surveys implemented in Africa, 2010–2019a

The red line denotes the best estimate and the blue shaded areas are the 95% confidence intervals.
a Surveys in the Gambia and Rwanda were restricted to only three age group categories because the number of survey cases was low. Bacteriologically confirmed TB cases could not be verified for the United Republic of Tanzania, so the prevalence of smear-positive pulmonary TB is shown instead.

Fig. 1.4.5 Estimated age-specific prevalence of bacteriologically confirmed pulmonary TB for surveys implemented in Asia, 2007–2021a

The red line denotes the best estimate and the blue shaded areas are the 95% confidence intervals.
a Results from recently completed surveys in Cambodia and Timor-Leste are pending.


As transmission declines, more incident cases arise from old (remote) rather than recent infection. Therefore, a pattern in which prevalence increases with age suggests that transmission is falling. It is encouraging that prevalence surveys indicated that transmission is potentially declining in many Asian countries and in several African countries (e.g. Ghana, Lesotho, Malawi, Mozambique, Rwanda and the United Republic of Tanzania). Elsewhere, surveys suggested considerable community transmission; peaks in many African countries in the age groups 35–44 or 45–54 years also reflect the impact of the HIV epidemic.

A striking finding across all surveys was the much higher burden of TB disease in men compared with women (Fig. 1.4.6). The male to female (M:F) ratio of bacteriologically confirmed pulmonary cases in surveys completed in 2007–2021 ranged from 1.2 (in Ethiopia) to 4.5 (in Viet Nam); in most countries it was in the range 2–4. These findings mean that men typically account for about 66–75% of the burden of TB disease in adults.

Fig. 1.4.6 The male to female ratio of bacteriologically confirmed adult TB cases detected in prevalence surveys implemented 2007–2021a

a Due to laboratory challenges during the survey in the United Republic of Tanzania, it was only possible to directly estimate the prevalence of smear-positive (as opposed to bacteriologically confirmed) pulmonary TB. Results from recently completed surveys in Cambodia and Timor-Leste are pending.


Ratios of prevalence to notifications (P:N, expressed in years) suggest marginally higher detection and reporting gaps in Asia compared with Africa, and lower detection and reporting gaps among women compared with men (Fig. 1.4.7, Fig. 1.4.8). The combination of a higher disease burden in men and larger gaps in detection and reporting indicates a need for strategies to improve access to and use of health services among men (9).

Fig. 1.4.7 The prevalence to notification (P:N) ratio of adult TB cases in prevalence surveys implemented 2007–2021a

a The P:N ratio is for smear-positive pulmonary TB, except for Bangladesh, the Democratic People's Republic of Korea, Kenya, Myanmar (2018), Namibia (2018), Uganda, Viet Nam (2017) and Zimbabwe where it was based on bacteriologically confirmed pulmonary TB (following updates to WHO guidance on case definitions used for TB surveillance published in 2014). Prevalence estimates are from a cross-sectional survey, and therefore only represent one point in time. Notification data are from the main year of the survey. Results from recently completed surveys in Cambodia and Timor-Leste are pending.

Fig. 1.4.8 The prevalence to notification (P:N) ratio by sex for adult TB cases in prevalence surveys implemented 2007–2021a

a The P:N ratio is for smear-positive pulmonary TB, except for Bangladesh, the Democratic People's Republic of Korea, Kenya, Myanmar (2018), Namibia (2018), Uganda, Viet Nam (2017) and Zimbabwe where it was based on bacteriologically confirmed pulmonary TB (following updates to WHO guidance on case definitions used for TB surveillance published in 2014). Prevalence estimates are from a cross-sectional survey, and therefore only represent one point in time. Notification data are from the main year of the survey. Results from recently completed surveys in Cambodia and Timor-Leste are pending.


A large proportion of survey participants with bacteriologically confirmed pulmonary TB did not report symptoms during screening and were only tested for TB based on their chest X-ray results. This proportion was higher in Asian countries compared with African countries, and varied from 30% in Malawi to 86% in Myanmar (Fig. 1.4.9). The classification and natural history of these individuals, their contribution to overall transmission at the population level, and implications for case-finding and treatment warrants further exploration (10–15). For more information about “asymptomatic TB” please refer to the featured topic on asymptomatic TB.

Fig. 1.4.9 Percentage of people detected with bacteriologically confirmed pulmonary TB detected in national TB prevalence surveys who did not report symptoms during screening, 2007–2021a

a Results from recently completed surveys in Cambodia and Timor-Leste are pending.


Data about health care seeking behaviour among people who reported symptoms that met survey screening criteria are available for 27 countries. Of those who had sought care prior to the survey, most initially sought care at public health facilities; this was followed by pharmacies, a mix of sources classified as “other” (e.g. traditional healer) and private health facilities (Fig. 1.4.10).

Fig. 1.4.10 Place of initial care seeking among survey participants who reported symptoms that met screening criteria and had sought care, 2007–2021a

a Results from recently completed surveys in Cambodia and Timor-Leste are pending. Results were not available from surveys conducted in Namibia, the Philippines (2007) and Thailand.


About half of survey participants who reported symptoms that met survey screening criteria had not sought care for their symptoms (Fig. 1.4.11). Many survey participants regarded their symptoms as not important enough to seek assistance, or faced geographical or financial barriers to accessing care. Addressing barriers to prompt diagnosis (e.g. the costs of accessing care, insufficient availability of rapid diagnostic tests at the places where people initially seek care, insufficient health care worker awareness about TB) is essential for the prompt diagnosis and treatment of people with TB.

Fig. 1.4.11 Percentage of survey participants who did not seek care for their symptoms, 2007–2021a

a Results from recently completed surveys in Cambodia and Timor-Leste are pending. Results were not available from surveys conducted in Namibia, the Philippines (2007) and Thailand.


For more information

A WHO publication provides full details about the results and lessons learned from the 25 national surveys implemented between 2007 and 2016 (4). This includes additional information about how repeat surveys have been used to assess trends (in Cambodia, China and the Philippines) and to measure the level of underreporting and to take corrective actions (in Indonesia). In addition, regional syntheses of survey results and lessons learned are available in journal articles (16, 17). Country case studies to showcase the results from surveys and how they have been used have also been included in previous editions of the Global TB Report (e.g. Myanmar and Viet Nam in 2019 (18), the Philippines in 2017 (19), Indonesia in 2015 (20), Nigeria in 2014 (21)).

A WHO publication in May 2023 discusses the diagnostic algorithms to be used in future surveys (implemented from 2023 onwards) (22). It provides the basis for recommendations on diagnostic algorithms included in the third edition of WHO guidance on national TB prevalence surveys which will be published later in 2024 (3).

 

References

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