At the time of the initial random blood glucose test 82% of 1787 patients who screened positive for type 2 diabetes, 81% of 2583 patients who screened negative, and 54% of 484 control participants responded. To ensure that data at this time captured the impact of the random test only, 310 questionnaires from screen positive patients that were completed or returned after the date of the second test were excluded, providing amended response rates of 65% for the patients who screened positive and 74% for the screened group (participants who screened positive plus participants who screened negative).
Response rates at 3-6 months were 66% among screening attenders, 18% among non-attenders, and 47% among controls. At 12-15 months response rates were 67%, 11%, and 40%. To ensure that analysis at these times captured the impact of the oral glucose tolerance test result, questionnaires received from attenders of this test before they had received their result (3-6 months, n=10; 12-15 months, n=4) were excluded.
The screening and control groups were comparable at baseline on the measures used to calculate the diabetes risk score17
and for practice size (table 1).
Table 1 Baseline characteristics of screening and control participants. Values are percentages (numbers) of participants unless stated otherwise
Impact of being invited to screening
At the time of the random test no significant differences were found between the screening attenders and control participants on any of the five outcome measures—for example, for state anxiety, difference in means −0.53 (95% confidence interval −2.60 to 1.54), P=0.62; table 2: no data were available on non-attenders at this time. At 3-6 months and 12-15 months no significant differences were found between those invited for screening (attenders and non-attenders) and control participants—for example, for state anxiety, difference in means at 3-6 months 1.51 (−0.17 to 3.20; P=0.10) and at 12-15 months 0.57 (−1.11 to 2.24; P=0.52, table 3). Thus the hypothesis that being invited to a screening programme has an adverse psychological impact was not supported.
Table 2 Differences in outcome between screening attenders and control participants, and between participants who screened positive and those who screened negative (random blood glucose test), at initial time point*. Values are means (standard deviations) (more ...)
Table 3 Differences in outcome between screening and control participants at 3-6 months* and 12-15 months†. Values are means (standard deviations) unless stated otherwise
Immediate impact of initial screening test results
After the initial test the participants who screened positive reported significantly poorer general health (difference in means −0.19, −0.25 to −0.13; P<0.001), higher state anxiety (0.93, −0.02 to 1.88; P=0.05), higher depression (0.32, 0.08 to 0.56; P=0.01), and higher diabetes specific worry (0.25, 0.09 to 0.41; P=0.002) than participants who screened negative (table 2). No significant difference was found on general anxiety. Thus, the second hypothesis, that participants who screened positive would have worse outcomes than those who screened negative, was supported for four of the five outcome measures. The effect sizes were, however, small.
Impact of a recent diagnosis of type 2 diabetes
A significant linear trend across the subgroups of screening attenders was found on two measures (table 4). At 3-6 months self reported health declined across groups according to the number of tests before testing negative, with the poorest general health reported by those testing positive at the final test—that is, those with newly diagnosed type 2 diabetes (P=0.047). The effect was no longer evident at 12-15 months. Secondly, the more screening tests that a participant underwent before testing negative, the higher the diabetes specific worry reported at 3-6 months and 12-15 months (P=0.001 in both cases). No significant trend was found across groups for the anxiety and depression measures. Thus, support was limited for the hypothesis of a dose-response effect of length of involvement with the screening programme on psychological costs.
Table 4 Trends across four subgroups of screening attenders, at 3-6 months and 12-15 months since initial random blood glucose test. Values are means (standard deviations) unless stated otherwise
Those invited for screening included two further groups (table 5): screening non-attenders (for the initial test) and dropouts (participants who tested positive at the initial test but did not attend for further tests and so never received their final result). Both had poor response rates (18% and 38% at 3-6 months, 11% and 37% at 12-15 months). Compared with screening attenders, non-attenders had significantly higher scores on diabetes specific worry at 3-6 months (difference in means 0.26, 0.01 to 0.50; P=0.04) and 12-15 months (0.35, 0.03 to 0.66; P=0.03), but the effect sizes were small. Participants who screened positive at the initial test but who dropped out were compared with those who screened positive at the initial test but attended for subsequent tests. Dropouts had significantly poorer self reported health at 12-15 months (−0.26, −0.46 to −0.06; P=0.01) and significantly higher diabetes specific worry at 12-15 months (1.25, 0.66 to 1.83; P<0.001), a medium sized effect.
Table 5 Differences in outcome between screening attenders and non-attenders (for random blood glucose test) and, within screen positive group (random blood glucose), between those who attended all subsequent tests and dropouts, at 3-6 months and 12-15 (more ...)
Sensitivity to missing data
The analysis at the initial test included only those participants who had screened positive and who had completed or returned their questionnaire before the second test, but included all responders who screened negative at the initial test. To mirror the exclusion criterion applied to the screen positive group, the analysis was repeated with a reduced screen negative group, excluding those who took a long time to return the questionnaire (matching the mean return time of the screen positive group). The only change in conclusion from this further analysis was that participants who screened positive scored higher than those who screened negative on state anxiety (difference in means 1.22, 0.25 to 2.18; P=0.01). The effect size remained small.
Completion rates for all items constituting the hospital anxiety and depression scale and measures for worry about developing diabetes were high (97%, 98%, 94%), but only 76% of participants completed all items constituting the state anxiety measure. At the initial test non-completers of the state anxiety measure had higher mean general anxiety (hospital anxiety scale) than completers, by 1.36 units in controls and by 0.79 units in participants who screened negative and 0.62 units in those who screened positive, with the same completion rate across groups. Inclusion of non-completers for state anxiety would therefore be expected to reduce the difference in scores between the group who screened positive and those who screened negative (table 2).
To investigate bias arising from dropout between the initial test and 3-6 months, non-responders to the questionnaire at 3-6 months were compared with responders at the initial test. For each outcome measure non-response rates were similar across the three main groups from the initial test to 3-6 months (within 7%). Non-responders had a mean outcome worse than responders; this was greater in the control group and at a similar level in the groups who screened negative or positive (data not shown). If non-responders had responded similarly at 3-6 months the effect would be relatively to worsen the outcomes in the control group and reduce the evidence in favour of a psychological impact of screening.