Page 14 - DISINVESTMENT AND IMPLEMENTATION OF VISION SCREENING TESTS BASED ON THEIR EFFECTIVENESS

CHAPTER 1
non-compliance) or incorrect diagnosis. Research on amblyopia and strabismus prevalence in multi-ethnic populations in respectively Baltimore (BPEDS study)17 and Los Angeles & Riverside (MEPEDS study)18 showed a prevalence of manifest strabismus (age 6-71 months) of around 3.3% for whites, 2.1% for African American and 3.55% for Asian children. Amblyopia prevalence (age 30-71 months) varied from 1.8% for whites and Asian children to 0.8% for African American. The Sydney Paediatric Eye Disease Study19 found an amblyopia prevalence of 1.9% and no significant associations with low birth weight, preterm birth, maternal smoking, age, gender, ethnicity or measurements of socioeconomic status. Higher amblyopia prevalence rates of 3.1% were found in a rural town in Poland20 to even up to 5.6 % in an unscreened population in Germany21. Differences in prevalence can (partially) be explained by differences in criteria used to define amblyopia.
SCREENING
Definition of screening
The World Health Organization defines screening based on the Commission on Chronic Illness United States as “the presumptive identification of unrecognized disease in an apparently healthy, asymptomatic population by means of tests, examinations or other procedures that can be applied rapidly and easily to the target population”.22 Persons who probably have a (early-stage) disease are sorted out from persons who probably do not have a disease. Persons who were found by screening to probably have a disease must be referred for diagnosis and necessary treatment. A screening test is therefore not intended to be diagnostic and screening does not find all persons who have a disease.
For screening to be effective, in other words to give treatment to those with previously undetected disease and to avoid harm to persons who do not require treatment, Wilson and Jungner23 described the following principles:
- The condition screened for should be an important health problem - There should be an accepted treatment
- Facilities for diagnosis and treatment should be available
- There should be a recognizable latent or early symptomatic stage
- There should be a suitable test
- The test should be acceptable to the population
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