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AIMS: Diabetes UK estimates a quarter of UK cases of diabetes are undiagnosed; 750,000 people have undiagnosed diabetes in addition to 2.25 million with known diabetes, but research studies examining this are contradictory. The aim was to determine the prevalence of, and risk factors for, undiagnosed diabetes in the population of England aged > 50 years and to calculate the percentage of cases of undiagnosed diabetes.

METHODS: This was a cross-sectional study in a nationally representative sample of 6739 people aged 52-79 years from the English Longitudinal Study of Ageing (ELSA) 2004/2005. Diabetes cases were ascertained by self-reported doctor diagnosis of diabetes. A fasting plasma glucose measurement after a minimum of 8-h fast was available for 2387 (38% of the participants without diabetes). Undiagnosed diabetes cases were based on a fasting plasma glucose >or= 7.0 mmol/l.

RESULTS: The overall weighted prevalence of diabetes was 9.1%; 502 people (7.5%) had self-reported diabetes (9.0% of men and 6.0% of women); 36 (1.7%) had undiagnosed diabetes (2.6% of men and 0.8% of women). Of cases of diabetes, 18.5% were undiagnosed (22% in men, 12% in women). Significant risk factors for undiagnosed diabetes were male sex, higher body mass index, waist circumference, systolic blood pressure and triglycerides.

CONCLUSIONS: In 2004 the prevalence of undiagnosed diabetes, and the proportion of cases of diabetes that were undiagnosed, appear smaller than in previous studies. This is likely to be due to increased awareness of diabetes and improved clinical care resulting in many of those with previously undetected disease having been diagnosed.