A study into how diabetes mellitus is connected to the common diabetic complication retinopathy suggests that the guidelines for diagnosing diabetes could need reassessment. These are the conclusions of authors of an Article in this week's edition of The Lancet.
Diabetes will affect an estimated 380 million people by 2025. Both WHO and the American Diabetes Association diagnose diabetes on the basis of a person's blood "fasting plasma glucose" (FPG) concentration -- those with a value of 7.0 mmol/L or higher are classed as diabetic. Retinopathy -- small blood vessel damage to the eye -- is a common complication of diabetes, which can lead to blindness. Three major studies in the 1990s have shown that retinopathy signs were rare below an FPG threshold of 7.0 mmol/L, but that their prevalence increased substantially above it. However one of these studies assessed retinopathy from a direct clinical ophthalmoscopic examination, and the other two from one retinal photograph. None used the current gold standard in clinical trials -- multiple field retinal photographs. Furthermore, early reports from the Diabetes Prevention Programme suggest many people have signs of retinopathy below an FPG of 7.0 mmol/L.
Professor Tien Wong, Centre for Eye Research Australia, University of Melbourne, VIC, Australia, and colleagues looked at three further studies in which multiple field retinal photographs were used to define retinopathy -- the Blue Mountains Eye Study (BMES, Australia, 3162 people); the Australian Diabetes, Obesity, and Lifestyle Study (AusDiab, Australia, 2182 people); and the Multi-Ethnic Study of Atherosclerosis (MESA, USA, 6079 people).
The researchers found that the overall prevalence of retinopathy in the general population ranged from 9.6% to 15.8%. They found no evidence of a clear and consistent FPG threshold for the presence or incidence of retinopathy across the different populations. In fact, more than 60% of retinopathy cases were missed by the widely used diabetes cutoff of 7.0mmol/L -- since these patients had FPG levels below this limit.
The authors say: "We found no uniform FPG glycaemic threshold for retinopathy across different populations and poor performance of current FPG cutoffs in separating individuals with and without retinopathy, largely due to the much higher prevalence of retinopathy at low FPG concentrations than previous studies reported."
They conclude: "These findings could help unify the understanding of the risk of complications from diabetes, suggesting that both macrovascular (large vessel) and microvascular (small vessel) complications do not seem to respect a glycaemic threshold. These findings further question the validity of the current WHO and American Diabetes Association approach of using retinopathy to derive FPG thresholds for diagnosing diabetes, and point to the need to revisit current diagnostic criteria for diabetes."
In an accompanying Comment, Drs Quresh Mohamed and Alison Evans, Cheltenham General Hospital, Cheltenham, UK, say that while current diagnostic criteria may be limited "the current cut-offs do seem to distinguish a group with substantially higher risk of harm." They also call for larger prospective studies into the relationship between impaired glycaemic control and various complications.
They conclude: "We perhaps should focus less on a single universal cut-off and instead target resources on the basis of standardised evidence-based individual risk scores in which measures of glycaemia are combined with other risk factors. But what would we tell our patients when they asked if they had diabetes" We are probably best sticking with what we know until a better alternative diagnostic tool becomes available."
Professor Tien Wong, Centre for Eye Research Australia, University of Melbourne, VIC, Australia T) +61 (3) 99298352 / +61 408 901 390 E) twong@unimelb.edu.au
Dr Quresh Mohamed and Dr Alison Evans, Cheltenham General Hospital, Cheltenham, UK T) +44 (0) 845 422 4192 E) queresh.mohamed@glos.nhs.uk / Alison.Evans@glos.nhs.uk
Journal
The Lancet