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How to assess the prevalence of trachoma
  1. H R Wright,
  2. H Vu,
  3. H R Taylor
  1. Centre for Eye Research Australia (CERA), Vision CRC, Victoria, Australia
  1. Correspondence to: Dr Heathcote R Wright CERA, Locked Bag 8, East Melbourne, Vic 8002, Australia;

Statistics from

TRA for intervention in higher prevalence areas, ASTRA for low prevalence areas

Trachoma is the world’s leading cause of infectious blindness, an estimated 84 million people have active trachoma and 7.6 million have trachomatous trichiasis.1 It is a disease of poor personal and community hygiene, affecting those living in the poorest conditions, and disappears as living conditions improve. Repeated or persistent infection with the obligate intracellular bacteria Chlamydia trachomatis results in the clinical syndrome of blinding trachoma. Trachoma progresses from inflammation of the upper tarsal conjunctiva to scarring; distortion of the eyelid causes trichiasis and eventual loss of vision secondary to corneal opacity after which blindness is essentially irreversible. The SAFE strategy developed by the World Health Organization (WHO) is effective in controlling blinding trachoma.2 It targets trachoma intervention at various stages of the cycle of disease: Surgery for trichiasis, Antibiotics for active trachoma, Facial cleanliness, and Environmental improvements. However, simple, reliable, and cost effective systems are needed to identify populations at risk of the blinding complications of trachoma and to assess the effectiveness of trachoma intervention programmes.


Population based prevalence surveys are the gold standard for estimating the prevalence of active trachoma and trachomatous trichiasis within a community. They have proved the mainstay of targeting and monitoring trachoma intervention; however, they are expensive, time consuming and may utilise resources that could be better spent on intervention programmes. The WHO has published guidelines advising how such a survey can be carried out in order to obtain a good random sample and provide accurate data.3 However, such surveys are relatively weak at distinguishing between a low level of trachoma and the absence of trachoma unless they have a very large sample size (figs 1 and 2). This is an important weakness in certifying that an area is free of disease.

Figure 1

 The 95% confidence interval (CI) for each survey method when the observed prevalence is 5% and 0%. Circles/squares show the observed prevalence, bars show 95% CI limits (actual value in parentheses), and arrows signify that there is no limit in this direction for this survey type.

Figure 2

 The 95% confidence interval (CI) for each survey method when the observed prevalence is 20% or 40%. Squares/diamonds show the observed prevalence, bars show 95% CI limits, arrows signify that there is no limit in this direction for this survey type.

Trachoma rapid assessment (TRA) developed by the WHO attempts to quickly, cheaply, and efficiently obtain the information needed to identify and prioritise areas for intervention programmes.4 It uses a two phase sampling technique to optimally bias the sample to the “worst places” within those communities most likely to have trachoma. At-risk communities are selected from within a region on the basis of existing trachoma information and known socioeconomic conditions. For this reason it can be confidently stated that if trachoma is not found in the “worst areas” it is most unlikely to be found anywhere else within that region. Unfortunately, some have tried to use data from TRA surveys to give prevalence estimates. TRA was explicitly designed not to yield prevalence data as it selects an “optimally biased” sample in order to detect trachoma if present.

Lot quality assurance sampling (LQAS) has also been trialled for the rapid assessment of trachoma.5 It has long been used in manufacturing and more recently by public health services, predominantly to evaluate service delivery particularly with respect to vaccination coverage. When used as a tool for the rapid assessment of trachoma prevalence the technique has been referred to as asymmetrical sampling trachoma rapid assessment (ASTRA). Children are examined until either a predetermined number of cases with active disease are identified (high prevalence) or a total of 50 children are sampled without the cut-off point being reached (low prevalence). Communities are categorised as low or high prevalence and the values of these categories can be adjusted by selecting the cut-off point at which sampling stops.


Results of several TRA field tests have been published some of which have compared TRA with prevalence survey results.6–9 They suggest that TRA is reasonably accurate in prioritising communities with higher levels of active trachoma. TRA did less well ranking communities with a low prevalence, although this is relatively less important as these communities were almost always assigned a low priority ranking. All studies reported that TRA was quicker and cheaper than a prevalence study. Two studies6,8 reported that there was an overemphasis on risk factor scores.

There is one published report of the effectiveness of ASTRA.5 The threshold was set at 14 and a maximum of 50 children aged 2–5 were identified for examination. They were able to accurately identify a community with a prevalence of ⩽20% with a sensitivity of 94% and a prevalence of ⩾40% with 95% sensitivity.


According to the WHO guidelines a community should receive mass antibiotic treatment when the prevalence of active trachoma is more than 10% among 1–9 year old children. Treatment should continue for at least 3 years and should not stop until the prevalence is below 5%.1 Prevalence surveys remain the gold standard and are necessary to monitor intervention programmes, but TRA and ASTRA have an important role. Deciding which survey method to use must take into account the aim of the survey, the anticipated trachoma prevalence, and any important local concerns.

When assessing an area to determine if and where an intervention programme should be implemented TRA provides the quickest and surest way of ascertaining whether trachoma exists or not. It will also assist in prioritising or ranking communities or areas for intervention. ASTRA could also be used but it may miss pockets of disease.

To monitor the progress of an ongoing trachoma intervention programme, ASTRA can give a broad brush guide of the prevalence of disease. However, the selection of some “sentinel” communities for repeat prevalence assessment may be preferred by some.

To certify the elimination of trachoma one would again turn to the targeted TRA method or else use a prevalence survey with a very large sample size.


When deciding on which survey technique to use it is important to consider the aim of the survey. TRA can accurately and rapidly prioritise communities for intervention in higher prevalence areas; however, ASTRA may be better in low prevalence areas. Once a community has been identified for intervention a prevalence survey, or possibly ASTRA could be undertaken to allow programme monitoring. Finally, in order to certify that a region is clear of trachoma TRA is the most efficient method.


This research at the Vision Cooperative Research Centre was partly supported by the Australian Federal Government through the Cooperative Research Centres Program.

Note in Proof

TRA for intervention in higher prevalence areas, ASTRA for low prevalence areas



  • Competing interests: none.

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