Aims To extrapolate, from the proportion of subjects with observable retinopathy at diagnosis of type 2 diabetes mellitus in routine clinical practice, the mean duration of undiagnosed diabetes.

Methods On 1 October 1999, there were 4313 patients with type 2 diabetes in the 41 participating practices in the Tayside region (registered with one of 166 GPs). 501 (12%; 95% CI 11 to 13%) patients were selected using a pseudo-random number allocation algorithm, and practice lists checked for recently deceased, non-residents (45 exclusions). Retinopathy was graded by validated slit lamp biomicroscopy and four-field stereo photography. Date of first diagnosis of diabetes was ascertained from the regional diabetes register created using multiple source data capture.

Results Of living Tayside resident patients, 295 from 456 invited type 2 patients (65%) were examined. 14.68% (95% CI 12.48 to 16.88%) were found to have retinopathy at diagnosis. Assuming a linear model, these data suggest that the onset of detectable retinopathy occurs 5.77 years (95% CI 4.6 to 7 years) before diagnosis. Comparison using the log rank test with survival to onset of sight threatening retinopathy/maculopathy in 291 patients with type 1 diabetes mellitus also examined from the same population cohort showed the 95% CIs of length of preclinical diabetes to be between 3.0 and 9.4 years.

Conclusion There is accumulating evidence to question the assumption of linearity as a model of choice. The authors' understanding of a distinct glycaemic threshold for retinal change is also overly simplistic and consequently the bounds of uncertainty concerning the preclinical duration of disease are considerable.