A single haemorrhage or microaneurysm would be classified as background retinopathy. Haemorrhages tend to resolve within a few weeks so won't be visible a year later. If no new retinopathy changes occur, the retina will appear "normal" at the next screening. Microaneurysms are small swellings on the capillaries in the retina. As retinopathy progresses, after about 3 years or so, the capillary shuts down and the microaneurysm is no longer visible. Again, if no new retinopathy has occurred, the retina will appear "normal" at the next screening. When someone is diagnosed with Type 2, they may already have retinopathy from years of undiagnosed diabetes. After diagnosis, an improvement in diabetic control can reduces the rate at which new retinopathy changes form.
Other possibilities, include:-
- data entry error (the grader ticks the wrong box)
- "looked but did not see" error (pathology is present but the grader doesn't spot it)
- differences in interpretation of the same feature (a pigment spot can look like a retinopathy change if the camera flash is bright)
Although the Diabetic Eye Screening Programmes in the UK have quality assurance measures (all results where retinopathy is reported will have been seen by at least 2 graders and, in addition, every grader will have a proportion of their "normal" results sampled and re-graded to make sure they are not habitually under-grading) the main aim of the screening service is to detect "sight-threatening" retinopathy rather than background retinopathy.