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Identifying a good screening test: sensitivity, specificity and coverage

Characteristics of a good screening test for diabetic retinopathy - high sensitivity, specificity, positive predictive value and high coverage.
Selecting the correct screening test
for a disease is affected by a number of factors: - The test must be easy to administer to people. - Cause minimal discomfort. - Be reliable - that is, consistently give the same result. - Be valid that is be able (by using key markers) to distinguish disease and non-disease states. - And, it must be affordable to the health service. A detailed examination of the retina, at the back of the eye, is the key procedure when screening for diabetic retinopathy. Various examination methods are available, from ophthalmoscopy to digital retinal imaging - with or without pupil dilation.
Whichever examination method is selected: Screeners must be able to find all cases of retinopathy correctly without causing unnecessary anxiety to the person being screened.
And, the health system must be able to: - Pay for, and manage the purchasing of, screening equipment and; - Train personnel to use and maintain it. We can use a 2 by 2 table to plot the presence of diabetic retinopathy against the ability of the test to detect the condition correctly. A good screening test for DR must have the following 4 key characteristics. - A high sensitivity that is the test must correctly identify all cases of retinopathy (known as true positives). - A high specificity the test must minimise falsely identifying cases as having retinopathy (false positives).
The test must also have a high positive predictive value ensuring a high probability that each person with a positive screening test truly has retinopathy. - And finally, the test must achieve high coverage so that everyone at risk of retinopathy (i.e. people with diabetes) is tested. Lets look at a hypothetical example to understand why these characteristics are important. In our population of 100 people with diabetes, 10 have retinopathy (these are known as cases). If all 100 people are screened, we have achieved high coverage. The perfect screening test will correctly detect all 10 cases (100% sensitivity) and identify all of the remaining 90 people as being free of retinopathy (100% specificity). Now lets see how an imperfect screening test performs.
This time, even when we ensure everyone is tested (high coverage) the test is only able to pick up 7 out of 10 cases correctly. Sensitivity is only 70%.
The test also incorrectly identifies 5 cases (false positives).
Only 85 out of the 90 people without retinopathy are correctly identified. Specificity is now 94%.
The predictive value of this screening test is low. Only 7 out of the 12 people (or 58%) identified by this test as having diabetic retinopathy actually have the disease. So this test is poor and not appropriate for scale up for screening a real population of people with diabetes. In practice, even with a test with high sensitivity and specificity, screening programmes need to achieve good coverage to perform well. In our example here, if half of the population is not screened, perhaps because of lack of resources, we will miss 4 cases who do not even get a chance to be detected.
In the group which is examined, the test correctly picks up 4 out of the 6 cases giving us a sensitivity of only 67% and making this a poor test. To deliver a high-quality screening programme, we have to consider both the key markers of a good test and how it will be implemented.
The balance we need to find is a test that: - Is good - has a high sensitivity and high specificity. - Is acceptable to the people being tested. - Can achieve high coverage - can be delivered to the whole eligible population. - And can be conducted repeatedly over regular intervals for example annual screening of the whole at risk population. To achieve these requirements, investment and protocols must be put in place.
The St Vincents Declaration of 2005 suggests that when establishing systematic screening,
programmes should aim to reach: a sensitivity of more than 80%; a specificity of more than 90%; and an acceptable coverage of at least 80%. For example, the English national screening programme, which began in 2003, reached 82.8% of the population by 2016. The selected test of 2 field mydriatic photography achieved a sensitivity of about 88% and a specificity of just over 86%, with 3.7% of images ungradeable. As camera technology develops, these rates are likely to improve. To maintain good coverage, the English screening programme closely monitors acceptance of screening as part of its performance criteria.
In summary. To be selected for a screening programme,
screening tests should: - Have a high sensitivity - to correctly find all cases of diabetic retinopathy - Have a high specificity - to reduce overloading the system with false positives and causing unnecessary anxiety amongst people incorrectly identified as having diabetic retinopathy - And achieve high coverage - to ensure everyone at risk is screened The test also has to be acceptable to the people with diabetes being screened and affordable for the health provider.
“The screening process is like passing people through a sieve. The holes in the screening sieve are a certain size that will catch some people and allow others to pass through. A screening test is designed to catch people who are at risk of a disease (it must be very sensitive) and allow those not at risk to pass through (it must be very specific).
Sometimes people will get stuck in the sieve who will turn out not to be at risk i.e. false alarms. Others will pass through the sieve despite being at risk i.e. missed cases (false negatives). Everyone picked up in the sieve will go on for more testing to determine if they have the disease and need treatment.”
Angela Raffle, Public health consultant to England’s national screening programme (Stolz & Pill, 2016)

The idea of a quick test to determine if a person has a disease or not is very appealing but no test can give the perfect “yes” or “no” answer. It remains important that the selected screening test is accurate and appropriate for the local population. Diabetic retinopathy (DR) screening programmes must prepare to manage each possible outcome of a screening test:

  • False positives: Provide further assessment and reassure these patients who have been initially incorrectly identified as having DR
  • False negatives (missed cases): Provide proper training for the screening team and quality assurance monitoring to reduce the number of missed cases. The unfortunate outcome for these patients is that they suffer irreversible loss of vision if treatment is delayed.
  • True positives: Implement good referral systems to provide those patients who are correctly identified as having DR with prompt and relevant treatment.
  • True negatives: Inform and empower patients about the importance of screening to encourage them not to become complacent and continue regular attendance.

The video on this step describes the characteristics of a good screening test. As you watch, consider why screening programmes might struggle to achieve high coverage for screening?

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