The MRC trial of the assessment and management of older people in the community is a large cluster randomised trial taking place in 106 general practices from the MRC General Practice Research Framework.7
The practices in the study were selected to be representative of the mortality (SMR) and Jarman scores of general practices in Britain (England, Wales, and Scotland). The aim of the trial was to compare the cost effectiveness of different methods of assessment and management of older people in the context of the 1990 contract of service which required general practitioners to offer an annual health check to patients aged 75 years and over. The study compared two different types of multidimensional assessment (targeted versus universal) and two different management models (primary care team versus multidisciplinary geriatric evaluation team). Randomisation was at the practice level and stratified by SMR and Jarman score. All patients aged 75 years or over on the general practitioner list were invited to participate in the trial, unless they were in long stay hospital or nursing homes, or were terminally ill.
People in the 53 practices allocated to the “universal” arm of the trial were given a visual acuity test as part of a detailed health assessment by the practice nurse. Visual acuity was measured at 3 metres with a Glasgow acuity chart which measures the minimal angle of resolution on a logarithmic scale (logMAR).8
Binocular vision was measured first, followed by vision in the right and left eyes. All vision measurements were conducted with usual spectacle correction. People with visual acuity of 0.5 or more in either eye (equivalent to less than 6/18 Snellen acuity) were retested with a pinhole occluder. If vision did not improve to less than 0.5, and the cause of visual loss had not previously been investigated, the person was referred to an ophthalmologist. If vision improved to less than 0.5, the patient was advised to see an optometrist. Visual impairment was defined as presenting binocular acuity of less than 6/18 (logMAR score 0.5 or more). In 49 practices, the cause of visual impairment was assessed by medical record review.9
The trial and additional data collection on causes of visual loss were approved by the relevant local research ethics committees.
People with AMD causing visual impairment were considered as “cases” and compared to a “control” group. There were two different options for selection of the control group. Firstly, to compare people visually impaired due to AMD with the rest of the MRC trial study population. This control group would include people visually impaired as a result of other causes and people not visually impaired. The second option considered was to compare people visually impaired due to AMD with people with good vision (that is, visual acuity of 6/6 or better).
The signs and symptoms of AMD form a continuous spectrum. Dichotomising the disease, as in many other conditions, is essentially arbitrary. In this study, relatively severe AMD cases were selected because a cut-off point of visual acuity worse than 6/18 was used to identify them. It is likely that a small proportion of people with vision worse than 6/6 and better than, or equal to, 6/18 will have AMD and a larger proportion will have early age related maculopathy (ARM)—that is, drusen and pigmentary changes putting them at increased risk of developing AMD.10
For this reason, in order to minimise the number of controls who have AMD or ARM, a control group of people with good vision—that is, binocular visual acuity of 6/6 or better, was selected.
Smoking history was ascertained using an interviewer administered questionnaire. The questions used came from the Whitehall study.11
Participants were asked whether they smoked currently. For people who responded no, they were asked whether they had ever smoked cigarettes. Age smoking started and stopped was also elicited and the number of cigarettes (ounces (g) tobacco) smoked a day. One ounce (28 g) of tobacco was assumed to correspond to 30 cigarettes.
The following variables were created:
- Smoking status 1=never smoked 2=ex-smoker 3=current smoker.
- Pack years were calculated from the number of years participants had smoked, multiplied by the usual daily cigarette equivalent intake, and divided by 20. This gives a measure of the lifetime exposure dose received.
- The number of years since stopping smoking was calculated from the current age minus the age stopped smoking. People who were still smoking had a value of 0; people who had never smoked were excluded from this variable.
The following confounding factors were considered because they have been reported in other studies as putative risk factors for AMD: socioeconomic status, alcohol consumption, cardiovascular disease and its risk factors. Data were not available on antioxidant micronutrient intake, exposure to light (visible or ultraviolet), and family history of AMD.
All regression analyses took account of the extra variation introduced by the cluster design of the study using the “svy” commands in Stata version 8.0 (Stata Corporation, TX, USA). “Svy” commands calculate robust standard errors using the “linearisation” variance estimator (based on a first order Taylor series linear approximation).