What is Prevalence Survey?
The prevalence of TB disease is the number of TB cases that exist in the population at a given pointin time. It is usually reported as the total numberof prevalent cases in a country, or as the numberof prevalent cases for a given unit of population(for example, the number of cases per 100 000population). Prevalence surveys are cross-sectional and population-basedsurveysorandomsampleofthepopulationin which the number of people withTB diseasein the survey sample is measured. All survey participantsare screened using interviews and chest X-rays. Sputum samples are then takenfrom all those with abnormal chest X-rays and/or symptoms suggestive of pulmonary TB. Sputumsamples are tested in laboratories to identify which individuals have bacteriologically-confirmedpulmonary TB (that is, Xpert MTB/RIF positive). Thepercentage of people with active TB in the populationat any given time is relatively low (lessthan 1% even in countries considered to have ahigh burden of TB). For this reason, the samplesizes required to estimate the prevalence of TB as opposed to infection, which has traditionally been measured usingtuberculinskin-testsurveys. Disease with reasonable accuracy are typically in the range of 30,000–100,000 people. Given the diagnostic technologies currently available and suitable for use in the context of apopulation-based survey, prevalence surveys focus on the measurement of pulmonary TB in adults. Surveys cannot yet be used to measure the prevalence of extra-pulmonary disease inadults or the prevalence of TB disease in children.
Why is Prevalence Survey Important for Nepal?
Surveys of the prevalence of TB disease are important for four major reasons.
The first and most obvious reason for conducting a survey is to obtain a direct measurement of the absolute burden of disease caused by TB. This is especially useful in Nepal, where there is considerable uncertainty about the number of TB cases and deaths, due to incomplete coverage or absence of surveillance systems.
The second reason for conducting a prevalence survey is to measure trends in the burden of diseasein the future, caused by TB.Repeat surveys conducted with an interval of around five or moreyears allow direct measurement of whether the burden of TB is stable, increasing or decreasing. Theycan also be used to assess the impact of efforts of a TB control program.
The third reason for carrying out the prevalence survey is to take the opportunity to understand the health seeking behavior of the general population for TB sysmtomps.
The fourth reason is to identify health service (TB services) utilization practice of TB patient (from participants who have / had TB disease).