Identifying a good screening test: sensitivity, specificity and coverage

“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?