Principles of screening
Screening is a public health service offered to people in a defined population (for example, preterm babies) who are at risk of a disease or condition for which earlier detection and management lead to better outcomes. Those eligible are offered a screening test to detect the disease early. Individuals who fail the screening test then undergo further investigations to confirm whether they have the disease or condition, such as sight-threatening retinopathy of prematurity (ROP), and are then offered treatment.
The aim of screening is to reduce the risk or impact of a disease or condition in a defined population. However, before screening is implemented within any health system, three main screening criteria must be carefully considered:
- Characteristics of the disease or condition: Is the disease or condition a public health problem?
- A suitable screening test: The screening test should be simple, safe, acceptable and valid and be implementable within the health system.
- Health service considerations: High quality healthcare for the disease or condition must be available.
1. Is the disease or condition a public health problem?
To answer this question requires:
- An understanding of the epidemiology of the disease or condition in the local setting.
- An understanding of the natural history of the disease or condition.
- A clearly defined target population based on the evidence.
For ROP screening: The epidemiology and natural history of ROP is well understood. However, at a local level it is important that evidence on the incidence of ROP and the population at risk of sight-threatening ROP is obtained or estimated before screening is implemented. This information is essential for defining the target population to be screened e.g by birth weight or gestational age.
2. What is the screening test and how will it be implemented?
To answer this questions requires:
- An accurate screening test, with high diagnostic test accuracy, such as high sensitivity and specificity, which is reliable and reproducible in the local setting.
- Available, accessible and adaptable post-screening pathways to confirm the diagnosis with referral for management, if needed.
For ROP screening: At the moment, most “screening” for ROP is undertaken by an ophthalmologist who examines babies at risk. As they make a clinical diagnosis at each examination, this is not screening in the true sense of the word. However, this is the term used internationally. True screening for ROP would entail a simple test for sight-threatening ROP, followed by clinical examination to confirm the diagnosis. At the moment retinal imaging with initial image grading by the person taking the image, with confirmation by an ophthalmologist does fulfil the requirements of a screening test.
3. What needs to be put in place to implement high quality healthcare for the disease or condition within a health system or programme?
To deliver high quality screening, treatment and follow up for a disease or condition requires:
- Adequate infrastructure, skilled health care personnel, equipment and organisation (including resource allocation) is in place.
- A coordinated service which is integrated into the health system where possible, for example, counselling, referral and treatment.
- A programme that is clinically, socially and ethically acceptable. Harms from over- or under-diagnosis or over- or under-treatment must be monitored and managed promptly. Quality has to be managed through regular evaluation.
- A screening service which is cost effective to implement, operate and sustain.
Screening programmes require continuous planning, monitoring and evaluation
When screening is introduced into a health system it is important that medical researchers and policymakers monitor results on the overall benefits and risks.
Implementing screening is a long term undertaking within a health system and requires careful planning. The cost of screening (human and financial) and the benefits of earlier treatment must be considered. After screening has been introduced into a health system it is important that medical researchers and policymakers monitor the overall costs, benefits and risks. Harm from over- or under-diagnosis or over- or under-treatment must be monitored and managed promptly. Quality has to be managed through regular evaluation.
Screening must be constantly monitored and revised: a better test may become available or new evidence may arise about the relevance and impact of the screening approach being used in the local setting.
A good screening programme arises from careful and continuous assessment on where and when it works.
For ROP screening: Some of the issues raised about ROP are discussed in a paper from India, which noted:
‘It is estimated that approximately 19 hours of ophthalmologists’ time is required to detect one case of threshold ROP (note: threshold disease was the indication for treatment before 2003). Although screening workload per child relatively decreases with increasing GA [gestational age] and BW [birthweight], the overall stress on the healthcare system due to the increased number of children under screening needs an astute evaluation. Screening by non-ophthalmologists and telescreening by wide‑field fundus imaging are paradigm changes in concept but need to be validated for agreement with the gold standard (screening by a trained retina specialist) in varied real‑life situations before these measures can be widely implemented.’ (Honavar 2019)
Questions for reflection
Consider the three main criteria for successful screening programmes - characteristics of the disease or condition, a suitable screening test and health service considerations. What is required to meet these criteria and successfully implement ROP screening in your setting? What would need to change? Share your reasoning in the Comments.
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