Screening for Diabetic Retinopathy
Screening - routine systematic testing of asymptomatic individuals - is generally considered to be justified if a disease is considered to be important, if it has a recognisable latent or early symptomatic stage, and if the natural progression from the preclinical to the clinical stage is well understood. There must be a suitable test or tests for identification of early disease, the test(s) must be acceptable to the population, and should be applied as part of a continuous cycle rather than as a one-off exercise. Furthermore, there must be an effective form of treatment, facilities for providing this treatment must be available, and there must be an agreed policy as to whom to treat. Last but not least, the treatment should be cost-effective. This entry describes the way in which screening people with diabetes for retinopathy satisfies these criteria, and explains how it may be delivered.
In order to understand the concept of screening for diabetic retinopathy, it is essential to define the term. The definition of screening that was adopted by the World Health Organization (WHO) in 1968 was ‘the presumptive identification of unrecognised disease or defect by the application of tests, examinations or other procedures which can be applied rapidly. Screening tests sort out apparently well persons who probably have a disease from those who probably do not.’
The UK National Screening Committee described screening as ‘a public health service in which members of a defined population, who do not necessarily perceive they are at risk of, or are already affected by a disease or its complications, are asked a question or offered a test, to identify those individuals who are more likely to be helped than harmed by further tests or treatment to reduce the risk of a disease or its complications’.
In 1989 representatives of government health departments and patients' organizations from all European countries met diabetes experts in St. Vincent in Italy. This meeting led to the ‘St. Vincent Declaration’, a set of recommendations to improve the medical care of patients with diabetes. The St. Vincent Declaration set a 5-year target to reduce new blindness due to diabetes by one third or more. This was the catalyst that encouraged the development of screening for diabetic retinopathy in Europe.
The four UK nations were the first countries in the world to introduce systematic national screening programmes for diabetic retinopathy.
Is screening for diabetic retinopathy appropriate?
The WHO criteria for screening outlined basic principles and is used to assess the suitability of any specific screening programme.
These principles fall into the four areas that must be considered
- Knowledge of disease:
- Knowledge of the test
- Treatment for the disease
- Cost considerations
So does screening for diabetic retinopathy fulfill these criteria?
KNOWLEDGE OF THE DISEASE
a. the disease should firstly be considered to be important.
Diabetic retinopathy has been previously recognized as being the most common cause of sight loss in the working age population in the UK. However a recent study of blind certification amongst working age adults in England and Wales found that for the first time in at least five decades, diabetic retinopathy/maculopathy is no longer the leading cause of certifiable blindness among working age adults in England and Wales. The main cause of blindness certifications was hereditary retinal disorders, while diabetic retinopathy/maculopathy accounted the second largest group. The authors postulated that the change may be related to improved glycaemic control and the influence of diabetic retinopathy screening programmes. Nonetheless diabetic retinopathy remains a major cause of potentially preventable blindness.
b. There must be a recognisable latent or early symptomatic stage.
There are numerous publications that show that sight-threatening retinopathy has a recognisable latent or early symptomatic stage. Patients with significant sight threatening diabetic retinopathy are often asymptomatic and therefore unaware that they are at risk. As such, screening for this disease is essential if sight-threatening retinopathy is to be identified early.
c. The natural course of the condition, including development from latent to declared disease, should be adequately understood.
There has been extensive research into and many publications on the natural history of diabetic retinopathy, including its progression from early to sight threatening disease, so the natural course of the condition is well understood.
KNOWLEDGE OF THE TEST
a. There must be a suitable test or examination for identification of the disease.
Any screening test to be utilised must be assessed on its sensitivity (i.e. ability to detect true positives) and its specificity (its ability to detect true negatives). For the purpose of screening for diabetic retinopathy, Diabetes UK has set standards of at least 80% sensitivity and 95% specificity for any screening program in the UK.
There are a number of screening modalities that may be considered for diabetic retinopathy screening
Direct Ophthalmoscopy. Ophthalmologists, optometrists, endocrinologists or GPs could in theory, carry out direct ophthalmoscopy, however the sensitivity of direct ophthalmoscopy has been found to be very variable.
Slit Lamp Biomicroscopy. Slit lamp biomicroscopy has been shown to be a potentially viable screening test with adequate sensitivity and specificity when carried out by well-trained individuals.
However for both direct ophthalmoscopy and slit lamp biomicroscopy, quality assurance is a challenge, since it requires repeated patient examinations to verify findings. A proportion of those who screen positive and of those who screen negative would need to be reexamined to ensure adequate reproducibility. Such repeat appointments would be inconvenient for patients and probably poorly attended.
Fundal Photography. Fundal photography has been shown to be an effective test for diabetic retinopathy screening, and is amenable to quality assurance. Hard copies of the images are retained and can be reviewed at any time.
Sensitivities for the detection of sight-threatening diabetic retinopathy of 87-100% have been found for trained personnel reading mydriatic (pupil dilated) 45° retinal photographs, with specificities of 83-96%. The results were similar between different types of personnel, including trained non-medical graders.
The use of mydriasis results in improved sensitivity for the detection of sight-threatening retinopathy, with fewer ungradeable images. The use of mydriasis currently varies among screening programmes within the UK.
Digital fundal photography has replaced 35mm film or Polaroid instant film prints. Digital imaging has many advantages, in particular, the images are easier to acquire and store. There are currently several software packages on the market specifically designed for grading of digital images of the retina in a screening setting.
b. The test must be acceptable to population.
Digital fundal photography is an acceptable, non-invasive test. The UK National Screening Committee has recommended digital retinopathy screening as the test of choice for diabetic retinopathy.
c. Case finding should be continuous (not just a 'once and for all' project).
Patients eligible for diabetic retinopathy screening should be identified on a systematic call-recall basis. At each screening cycle, all eligible patients are identified and offered a screening appointment. At the next screening cycle this process is repeated to ensure that all previously screened patients who remain eligible are invited back for repeat screening, and that any newly-eligible patients are identified and offered screening. The system aims to maximize the uptake of screening and to ensure that all eligible patients are offered screening.
TREATMENT FOR THE DISEASE
a. There should be an accepted treatment for patients with recognised disease.
For a disease to be suitable for screening, it must be one with an accepted and effective treatment. The mainstay of treatment for diabetic retinopathy is laser treatment. The efficacy of laser treatment been demonstrated in two large randomised clinical trials - the Diabetic Retinopathy Study (DRS) and the Early Treatment Diabetic Retinopathy Study (ETDRS). These trials showed that laser photocoagulation reduces visual loss in patients with proliferative diabetic retinopathy (PDR) and macular oedema by about 50% compared with no laser treatment. The ETDRS showed that early laser treatment was beneficial. More recently, novel therapies have been developed for the treatment of diabetic macular oedema.
Further detail can be found in Treatment of Diabetic Retinopathy
b. There must be facilities available for diagnosis of the disease and treatment must be available for those patients identified with the disease.
A screening test is not intended to be diagnostic. Persons with positive or suspicious findings must be referred for diagnosis and, if appropriate, treatment. Individual screening programmes must have access to an ophthalmology unit with adequate medical expertise and treatment facilities. Within the UK this should be achievable for all diabetic retinopathy screening programmes.
c. There must be an agreed policy concerning whom to treat.
The laser treatment protocol used in clinical practice is largely based on the combined findings of the DRS and the ETDRS  Additionally the National (UK) Institute for Clinical Excellence (NICE) have published recommendations on the use of Ranubizimab for treating macular oedema, and the Royal College of Ophthalmologists have published guidelines for treatment of diabetic retinopathy and diabetic macular oedema. All these resources are widely used in clinical decision-making regarding treatment.
A screening service must be cost effective. The UK programme for systematic screening for diabetic retinopathy has been found to be cost-effective in terms of sight years preserved compared with no screening.
In conclusion, assessment of diabetic retinopathy screening using WHO criteria makes a strong case that screening for diabetic retinopathy is indeed appropriate.
What does the current diabetic retinopathy screening provision look like in the UK today?
Screening is offered to everyone with diabetes who is aged 12yrs or more.
Patients are identified and invited for screening on an annual call-recall system.
There are different models of screening being used within the four UK nations. These include static screening units in NHS hospitals, mobile units that visit locations such as GP surgeries, and optometry-based services employing accredited optometrists.
Variations exist in the screening and grading protocols between the four UK nations. In general, digital photographs are captured and graded, and the results are sent to the General Practitioner and the patient. Patients are then either recalled for annual screening, or referred to hospital eye services, depending on results. In some programmes, patients may be invited for more frequent surveillance if appropriate.
All four UK nations use an accreditation package. This package provides a Level 3 Qualification in Diabetic Retinopathy Screening for screening staff, including photographers and graders.
Quality assurance (QA)
QA is essential for the maintenance of minimum standards and the continuous improvement of any screening programme. QA falls into two categories:
Internal QA -
Individual programmes have agreed QA standards to ensure safety and efficacy of the programme. QA standards should be measurable and reflect the programme’s objectives. For example, an objective may be to ensure photographs are of adequate quality. Other standards address areas such as grading quality, screening uptake, timescales in the screening pathway and patient outcomes. For each objective a minimum standard and ‘achievable’ higher standard are agreed. This allows for regular internal QA that can be compared year on year, and potentially compared with performance in other programmes.
External QA -
External QA is usually carried out by a visiting team to the screening programme to perform peer review. The UK National Screening Committee recommend that such visits should take place at least every 3 yrs. The objectives of an external QA visit are:
- To examine the performance of care affecting the quality of the screening programme
- To verify achievement of national minimum standards, and identify variance from these standards; to support professionals working in the programme in maintaining and improving standards
- To gain knowledge and expertise of best practice and disseminate this to all screening programmes
- To share experience and understanding of current issues in diabetic retinopathy screening, and contribute to national development
The UK National Screening Committee
- Advises Ministers and the NHS in the four UK countries about all aspects of screening.
- Supports implementation of screening programmes.
- Assesses the evidence for programmes against agreed criteria.
- Aims to ensure that screening programmes do more good than harm at a reasonable cost.
- Sets up practical mechanisms to oversee the introduction of new programmes in the English NHS and monitors effectiveness and quality of these programmes
A note on the limitations of screening
It must always be remembered that screening for diabetic retinopathy is intended to provide early identification of sight threatening disease, but cannot offer a guarantee of protection.
In any screening programme, there are inevitably false positive and false negative results. These can result in false reassurance for some patients. As such, screening must be presented and viewed as an exercise in as risk reduction.
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