Screening For disease

Question 1

Definition and purpose of screening

Answer 1

• Testing people who do not suspect they have a health problem (without symptoms), so as to:
• Reduce risk of future ill health
• by earlier detection and treatment
• Provide information
• to help make choices

Question 2

NHS national screening programmes

Answer 2

• Cervical cancer
• Breast cancer
• Bowel cancer
• Abdominal aortic aneurysms
• Antenatal and neonatal testing
• Diabetic eye disease

Question 3

Screening within other NHS programmes

Answer 3

• Health Check: screening adults aged 40-74 forearly signs of stroke, kidney disease, heart disease, type 2 diabetes or dementia
• Eg Cardiovascular risk factors

• National Diabetes Prevention Programme
• Based on glucose (or HbA1c) testing in Health Check
• Diagnosis of prediabetes => lifestyle interventions
• Diagnosis of diabetes => GP

Question 4

Components of a screening programme

Answer 4

• Register of eligible people
• System of invitation and recall
• Screening tests
• Confirmation of diagnosis
• Treatment or other interventions
• Information and support for patients
• Staff training
• Standards and quality assurance

Question 5

Screening can cause harm too

Answer 5

• Over diagnosis
• False positive tests
• Further testing can lead to more false positive tests
• False negative tests
• False sense of security
• Unnecessary treatment
• Might never have progressed to severe disease or death
• Costs of screening, further testing and treatment
=> There are always trade-offs between possible harms and benefits

Question 6

True and false test results
(Brief introduction to diagnostic test accuracy covered later in Research Methods)

Answer 6

Has disease. No disease

Screening tests positive: True (a) and false (b) positives
Screening tests negative: False (c) and true (d) negative

Sensitivity = A / (A+C) i.e. % of people with disease who test positive

Specificity = C / (B+C) i.e. % of people without disease who test negative

Question 7

Evidence about effectiveness of screening

Answer 7

• Randomised controlled trials (best)
• Time trends in disease incidence and outcomes
• Compared to countries or regions without screening
• Case control studies
• Is % screened lower in cases with disease than in controls without disease?
• Often biased by confounding
• Systematic reviews of evidence
• Modelling (combining a variety of evidence)

Question 8

Common sources of bias in screening evaluation

Answer 8

• Healthy screening effect
• People who take part tend to be healthier than those who don’t
• Length time bias
• Disease is more likely to be detected in people with longer lasting and slowly
progressive types of the disease => have better outcomes anyway
• Lead time bias
• Earlier detection makes duration of survival after diagnosis longer, even if treatment is ineffective

Question 9

Screening summary

Answer 9

• Screening is a common and important part of medical practice in general practice and many specialities
• It is an important part of public health
• Screening programmes are much more than just tests
• Screening is imperfect and can cause harm as well as benefits