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Systematic screening for diabetic retinopathy has been identified as a cost-effective use of health service resources. 1 - 4 The Health Technology Board for Scotland recommended a national screening programme using digital photography and a multi-level manual grading system ( fig 1 ), which is currently being implemented in Scotland. 5 Similar programmes are also underway in England, Wales and Northern Ireland.
With 161 946 individuals recorded on diabetes registers in Scotland, 6 manual grading is a resource-intensive activity. Current policy is implemented by capturing digital images at local screening centres, which are then sent electronically to one of nine regional grading centres. 7
However, a system of automated grading could provide cost savings to the NHS. Our research group recently developed and evaluated an automated grading system that can assess digital retinal images for quality 8 and the presence of retinopathy. 9 This system could thereby potentially replace manual level 1 grading. The purpose of this paper is to assess the cost-effectiveness of replacing this manual disease/no disease grading with the automated system, in the context of the three-level grading system used in Scotland ( fig 1 ). A decision tree model was developed to compare NHS grading costs and screening outcomes over a one-year period for these two alternative strategies.
METHODS
The model was developed and analysed using TreeAge Pro 2005 (Treeage Software Inc, Williamstown, MA, USA). Disease prevalence and screening efficacy parameters ( table 1 ) were all obtained from a study assessing the efficacy of automated and manual grading on a test set of 14 406 images from 6722 patients. 9 The cohort was almost entirely Caucasian, 55% of the patients were male, the median age was 63 (IQR 19) years, 88.5% were over the age of 45, and 52.8% over 65. These demographics are similar to those in the rest of Scotland.
Variables | Point estimate (95% CI) |
Prevalence variables |
|
Prevalence (normal cases) | 0.681 (0.670-0.692) |
Prevalence (mild retinopathy) | 0.266 (0.255-0.276) |
Prevalence (observable retinopathy/maculopathy) | 0.013 (0.011-0.016) |
Prevalence (referable retinopathy/maculopathy) | 0.040 (0.035-0.045) |
Technical failure rate | 0.082 (0.076-0.089) |
Efficacy of level 1 manual grading |
|
Detection rate for technical failures | 0.969 (0.951-0.981) |
Proportion of normal cases appropriately recalled | 0.920 (0.911-0.927) |
Detection rate for mild retinopathy | 0.819 (0.800-0.837) |
Detection rate for observable retinopathy/maculopathy | 1.000 (0.956-1.000) |
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