Page 63 - DISINVESTMENT AND IMPLEMENTATION OF VISION SCREENING TESTS BASED ON THEIR EFFECTIVENESS

The effect of omitting an early population-based vision screen in the Netherlands: A micro-simulation model approach
We simulated eight incidence curves with lower and upper estimates. A best-fit approximation for the incidence curve was performed during the fitting of the sensitivity, using the discrete quasi maximum likelihood method, comparing the observed values in the study with the predicted values (appendix figure). In this way, the most likely incidence curve was estimated.
To calculate the mean sensitivity, we assumed that all screens using the same combi- nation of tests (appendix table) had the same sensitivity. The model was run with 20 different values of sensitivity between 0% and 100%. We first estimated the mean sensitivity of the 6–24 months orthoptic screens. For each incidence curve, screening was simulated using the 20 values of sensitivity for the preverbal screens. The available subjects for that specific screen examination were obtained from the incidence curve. For each screen the screening test was simulated per subject, subsequently using one sensitivity out of the range. This resulted in a number of positively screened subjects per screen. For each incidence-sensitivity combination this was repeated 4000 times. Each time the simulated number of detected subjects for the preverbal screens was equal to the number of detected subjects in the observational study, the sensitivity used was implemented in an array. After the simulation, a mean sensitivity for that incidence curve was calculated, based on the sensitivity values in the array. The mean sensitivity for screens using the VA test was obtained in a similar way.
Overall sensitivity of the Dutch vision screening programme had been calculated as 73% in the RAMSES study (73 of the 100 detected amblyopia cases had been positively screened).5 For this calculation, sensitivity had been defined as the proportion of children with amblyopia who had a positive vision screening result at any point in time. Because 12 of the 73 amblyopia cases with a positive screening had been detected by the parents, but were positively screened later on, and in one case it was unclear whether this child visited the ophthalmology department after the positive test, in the current evaluation only the 60 screen-detected cases with amblyopia were used.
Using the incidence curves and the mean sensitivity per screen, we simulated the entire screening programme. With the estimated effect per detected case, we estimated the effect per screen. The effectiveness of the programme was calculated by summing the effect per screen. Starting with the scenario of the current vision screening programme, we subsequently calculated what the effectiveness of the screening programme would be after omission of the screen that was found to be least effective in the original simulation.
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