Lecture 35 - Screening Flashcards
What is screening
Screening is the widespread use of simple test to detect disease in an apparently healthy (asymptomatic) population
Screening programme
It is an organised system that uses a screening test among asymptomatic individuals to identify early cases of disease and improve outcomes
Diagnostic test vs screening test
A screening test empathises cost and safety, detecting possible disease cases for further testing, while a diagnostic test focuses on definitive diagnosis. A screening test is not a diagnostic text or a screening programme
Why is early detection through screening beneficial
Early detect aims to limit the consequences of disease through easy diagnosis and treatment, often improving prognosis
Key concepts of screening
- Aims to improve outcome, reduces mortality
- Screening programmes vs case finding (opportunistic screening)
- All screening programmes do harm; some can do good as well
- Screening is a pathway not a test
Screening programme pathway
The pathway includes health promotion, invitation, screening procedure, diagnosis, treatment or recall
Examples of NZ screening programme
- Antenatal screening for Down syndrome
- Breast screen Aotearoa
- National bowel screening programme
- National Cervical screening programme
- Newborn Metabolic screening programme
- Universal newborn hearing screening programme
When should we screen?
- Is the disease appropriate
- Is the test appropriate
- Would a programme be effective
- Consider benefits vs harms of screening
What factors determine whether a disease is appropriate for screening
- The seriousness of the disease
- Ability to alter course of the disease
- Prevalence of pre-clinical disease
- Lead time
Seriousness of disease
Screening is resource-intensive
So makes sense to screen for disease with potential severe consequences
Prevalence if pre-clinical disease
More efficient when high prevalence of preclinical disease
- Positive Predictive value
Lead time
Lead time is the extra time gained by detecting a disease early through screening before symptoms appear
ability to alter course of disease
- Disease is not detectable - first biological onset
- Screening may be of benefit - ideal to screen
- Usually diagnose - no benefit
Screening has to improve the length and/or quality of someones life
Intrinsic test properties - Is the test appropriate
Can measure accuracy with sensitivity and specificity
Sensitivity
Sensitivity is the proportion of people with the disease who test positive. High sensitivity reduces false negative
True positive/ (true positive + false negative)
Specificity
Proportion of people without the disease who test negative. High specificity reduces false positive.
True negative/ (false positive + true negative)
Which is the best, sensitivity or specificity
Increasing sensitivity may reduce specificity and vice versa. The balance depends on the consequence of false negatives vs false positives
How do you choose which one to maximise
- consider consequences of missing. cases versus false alarms
Sensitivity: - Detecting as many cases as possible important
- Costs or risks of next step not too high
Specificity:
Costs or risks of next step high
Predictive values
Measure test performance in a particular population - What proportion of people who test positive/negative do/don’t have disease
Positive predictive value (PPV)
Proportion of people who test positive and have the disease. Depends on the prevalence
True positive / (true positive + false positive)
Negative Predictive Value
Proportion of people who test negative and don’t have the disease. Depends on the prevalence
True negative / (false negative + true negative)
Screening programmes effectiveness - having the resources
Facilities and systems:
- Manage participation
- Cost and accessibility
- Quality control and monitoring
Treatment:
- Capacity to treat true positives
Cost effectiveness:
- Many people over long period
- Cost vs Benefit
Evaluation of screening programmes:
- Crucial to determine if screening programme actually leads to benefit
Benefits of screening programmes
- Potential for early detection and interventions
- Reassurance (true negatives)
- Improved health of population
Harms of screen programmes
- Increase in health inequities from unequal participation or treatment
- Physical - from complications and etc
- Psychological - from anxiety from waiting , distress from procedures and etc
- False positives - period of stress and uncertainty
- Financial - to individual
- Lead time bias
- Length
Lead time Bias
Lead time bias refers to the appearance of increase survive time simply due to earlier detection. Over diagnosed and / or over-treatment - may increase morbidity without reducing mortality
Length bias
Length bias occurs because screening is more likely to detect slower growing, less aggressive cases of disease, leading to a better prognosis than in faster growing cases, skewing results.
