de Fine Olivarius et al. BMC Public Health 2011, 11:80 http://www.biomedcentral.com/1471-2458/11/80 myocardial infarction and/or stroke causing hospitaliza- tion; and amputation of any part of leg or foot before or at the time of diabetes diagnosis. In questionnaires, patients gave information about whether they lived alone, their education, familial dispo- sition to diabetes, smoking habits, leisure time physical activity, angina pectoris, intermittent claudication, and global self-rated health. Definitions The decimal acuity of patients’ best seeing eye was used to categorise patients according to usual clinical and adminis- trative practice in Denmark: normal vision (≥ 0.5), moder- ate visual impairment (< 0.5 and > 0.1) or blindness (≤ 0.1). Cardiovascular disease was defined as history of myocardial infarction and/or history of stroke and/or angina pectoris and/or intermittent claudication and/or absent arterial pulses on both feet and/or amputation on the lower extre- mities. Peripheral neuropathy was defined as lack of a sense of pin prick and/or touch of cotton wool on at least one foot and/or absent patellar reflex on at least one knee. Assays Laboratory analyses were centralized and quality con- trolled. Fasting blood samples were analysed at Odense University Hospital. Throughout the study, haemoglobin A1c (HbA1c) was determined by the same ion-exchange, high-performance liquid chromatography method. Sam- ples from 100 blood donors (age 20-80 years, 33 men, 67 women) were analysed, and the reference interval (mean ± 2SD) was calculated to be 5.4-7.4%. Quality assurance was obtained with commercial control pre- parations from Bio-Rad. In October-December 1995, the mean (SD) of low (n = 24) and high (n = 29) control samples were 6.7 (0.31)% and 10.4 (0.63)%, respectively, resulting in coefficients of variation (CV = SD × 100/ mean) of 4.6% and 6.0%. Serum total cholesterols were measured enzymatically with cholesterol esterase- cholesterol oxidase-peroxidase reagent, and fasting serum triglycerides was determined enzymatically with a lipase-glycerolkinase-glycerol-3-phosphate oxidase- peroxidase reagent. Serum creatinine was determined by the Jaffe reaction. In this nationwide study a freshly voided morning urine sample was used instead of timed urine collection to determine renal involvement. Urinary albumin concentration was measured at Århus Univer- sity Hospital by a polyethylene glycol radioimmunoassay. Spot urinary albumin concentration is of value in pre- dicting both progression of renal involvement and increased mortality, as is albumin excretion rate [26]. Statistical analysis The influence of baseline characteristics on change in visual acuity over 6 years was investigated in linear Page 3 of 13 mixed models [27] using all available measurements of visual acuity from the best seeing eye. For each of the characteristics two models were constructed: one model where the change in visual acuity was described by a lin- ear time trend (intercept and slope for the average change in visual acuity during the study) that differed for each of the categories of the characteristic, and a second model where the time trend additionally depended on age and sex in a multivariable regression fashion. Within-patient correlation of the observations was modelled by a patient random effect. The effect of the characteristic on changes in visual acuity was sum- marized with the fixed effect intercept and slope for each of the covariate classes from the first model (i.e. excluding age and sex). Differences between the covari- ate categories in intercept and slope in the two models were tested with the appropriate F-tests. In the 5.8% (72/1241) of patients who had an eye operation in the best seeing eye during follow up, measurements of visual acuity made after the operation were excluded from the analyses. For the patients who were not moderately visually impaired or blind at baseline, incidence rates for these conditions were estimated as the ratio between (1) the number of patients with moderately impaired vision/ blindness at follow up and (2) the sum of follow up times. The follow up times were halved for patients who became moderately visually impaired or blind during follow up, and confidence intervals were calculated assuming Poisson-distributed occurrences of vision impairment. Simple comparisons were made with Krus- kal-Wallis tests and c2-tests. To account for multiple statistical testing, we applied the method of Benjamini- Hochberg [28] on all the four columns of p-values in Table 1, i.e. 4 × 28 = 112 p-values, and found that a sig- nificance level of 0.0179 was to be used in order to con- trol the false discovery rate at 5%. Accordingly, the nominal level of statistical significance was chosen to be p < 0.05, except in the mixed models in Table 1, where it was p < 0.01. Results Baseline characteristics At diabetes diagnosis median (IQR) age was 65.5 (56.0- 73.6) years, male/female ratio was 1.11 (653/588), and median diagnostic plasma glucose was 13.7 (10.7-17.0) mmol/l. The prevalence (95% confidence interval) of blindness and moderate visual impairment was 0.9 (0.4- 1.4) % and 5.4 (4.1-6.7) %, respectively (Table 2). AMD, other non-diabetic retinopathy and, above all, cataract were common. The ophthalmologists estimated that cat- aract was the most common cause of visual impairment except among the blind (Table 2). At diagnosis the eye doctors reported glaucoma in the best seeing eye in 18