Objective: To quantify the psychological impact of primary care based stepwise screening for type 2 diabetes.

Design: Controlled trial and comparative study embedded in a randomised controlled trial.

Setting: 15 practices (10 screening, five control) in the ADDITION (Cambridge) trial in the east of England.

Participants: 7380 adults (aged 40-69) in the top fourth for risk of having undiagnosed type 2 diabetes (6416 invited for screening, 964 controls).

Interventions: Invited for screening for type 2 diabetes or not invited (controls), incorporating a comparative study of subgroups of screening attenders. Attenders completed questionnaires after a random blood glucose test and at 3-6 months and 12-15 months later. Controls were sent questionnaires at corresponding time points. Non-attenders were sent questionnaires at 3-6 months and 12-15 months.

Main outcome measures: State anxiety (Spielberger state anxiety inventory), anxiety and depression (hospital anxiety and depression scale), worry about diabetes, and self rated health.

Results: No significant differences were found between the screening and control participants at any time-for example, difference in means (95% confidence intervals) for state anxiety after the initial blood glucose test was -0.53, -2.60 to 1.54, at 3-6 months was 1.51 (-0.17 to 3.20), and at 12-15 months was 0.57, -1.11 to 2.24. After the initial test, compared with participants who screened negative, those who screened positive reported significantly poorer general health (difference in means -0.19, -0.25 to -0.13), higher state anxiety (0.93, -0.02 to 1.88), higher depression (0.32, 0.08 to 0.56), and higher worry about diabetes (0.25, 0.09 to 0.41), although effect sizes were small. Small but significant trends were found for self rated health across the screening subgroups at 3-6 months (P=0.047) and for worry about diabetes across the screen negative groups at 3-6 months and 12-15 months (P=0.001).

Conclusions: Screening for type 2 diabetes has limited psychological impact on patients. Implementing a national screening programme based on the stepwise screening procedure used in the ADDITION (Cambridge) trial is unlikely to have significant consequences for patients' psychological health.

Trial registration: Current Controlled Trials ISRCTN99175498 [controlled-trials.com].