Page 68 - DISINVESTMENT AND IMPLEMENTATION OF VISION SCREENING TESTS BASED ON THEIR EFFECTIVENESS

CHAPTER 3
but our incidence curves also showed an increase in strabismic amblyopia after age 6 months and a higher incidence of refractive amblyopia in comparison with strabismic amblyopia. Williams et al. reported a prevalence of 0.7–3.4%, depending on whether the children had undergone preschool screening and the definition of amblyopia.10 We assumed equal sensitivities for all preverbal exams at 6–9, 14, and 24 months, as these screening examinations comprise the same tests. In the RAMSES study, however, as in real practice, these multi-component screening examinations were considered as one screen. It is possible that testing at an older age would have had a higher sensitivity, because of better cooperation and understanding of the tests. Also, if all separate screens are combined into one programme, this will give a higher sensitivity.
The low number of refractive amblyopia cases detected might be explained by the fact that strabismus causing amblyopia is often noticed by the parents, but a refractive error is not.
Complete and detailed observational data are useful to analyse the effectiveness of population-based screening programmes, but, because observational data is not dynamic, it is difficult to predict the effect of changes in a screening programme. Analysis is complex as the net effect of omission of one link in the chain is difficult to predict. Evaluation of alternative different screening strategies is difficult, as such studies are often expensive and time consuming. A micro-simulation model is dyna- mic, and can be used to evaluate different screening scenarios and compare their effectiveness. As our model was able to compute the mean number of detected true cases for each consecutive screen, it is well suited to evaluate a screening programme consisting of repeated screens. Further work is needed to validate and extend the model, so that it can be used to simulate screening programmes using different tests for each screen. With more input data, the model might be used as a general tool for evaluating the effectiveness of screening programmes with multiple screenings. As the input can easily be altered per disease specification, it could simulate screening for different disease types.
Further work is needed before the model is able to provide evidence for modifications of screening programmes, but it can be used as a tool to compare the effectiveness of different screening programmes, for instance in different countries in Europe, provided that detailed input data such as sensitivity of the tests used and background prevalence, are available.11 Since the completion of this microsimulation model study, we have started a disinvestment study, comparing two sequential birth cohorts of approximately 6000 children in a care region in the Netherlands. In the second birth
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