Lecture 35: Screening Flashcards
What is screening?
The widespread use of a simple test for a disease in an apparently healthy (asymptomatic) population
What is a screening programme?
An organised system using a screening test among asymptomatic people in the population to identify early case of disease in order to improve outcomes
What is a screening test?
A test, usually relatively cheap and simple, used to test large numbers of apparently healthy people to identify individuals suspected of having early disease who will then go on to have further diagnostic tests to confirm the diagnosis
Note: A screening test is not a diagnostic test or a screening programme
What is the difference between a screening test and a diagnostic test?
A screening test differs from a diagnostic test in that there is greater emphasis on cost and safety and less on definitive diagnosis
What is the purpose of detecting disease early?
Aim to limit the consequences of disease through early diagnosis and treatment - example of secondary prevention
What are the key concepts of screening?
- Aims to improve outcomes, usually to reduce 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
What are the steps of a screening programme?
- Health promotion initiatives
- Invitation
- Screening procedure
- Negative = back to recall and then to invitation
- Positive = sent for diagnosis and treatment, and then back to recall and invitation
What are some examples of NZ National Screening Programmes?
- Antenatal screening for Down syndrome and other conditions
- BreastScreen Aotearoa
- National Bowel Screening Programme
- National Cervical Screening Programme
- Newborn Metabolic Screening Programme
- Universal Newborn Hearing Screening Programme
What 4 aspects are considered for using a screen?
- Is the disease appropriate
- Is the test appropriate
- Would a programme be effective
- Consider benefits vs harms of screening
What 4 things should be considered when determining if the disease is appropriate for screening?
- Seriousness of disease
- Screening is resource-intensive so screening is used for diseases with potentially severe consequences - Ability to alter course of disease
- Critical point 1 = Disease is not detectable (between first biological onset and disease detectable)
- Critical point 2 = Screening may be of benefit (between disease detectable and symptoms appear)
- Critical point 3 = Usually diagnosed anyway – no benefit (between symptoms appear and outcome)
There has to be some effective therapy or treatment available - screening has to improve the length
and/or quality of someone’s life - Lead time
- Long lead time = greater chance of detecting disease early (greater time between disease being detectable and symptoms appearing) - Prevalence of pre-clinical disease
- More efficient when high prevalence of pre-clinical disease: Positive Predictive Value. Unless it’s a serious disease and cheap and easy to administer test
+ Knowledge of the natural history of the disease is important
Sorry it’s a long one but make sure you know the critical points on the graph slide 16 heheh
What 2 things should be considered when determining if the screening test is appropriate?
- Is the test accurate
- Ideally + test = + disease however, sometimes + test = - disease - Is the test acceptable and safe
How can the accuracy of a test be measured?
Sensitivity and specificity (maximise both in a screening) - Intrinsic properties of the test, measures what proportion of people with or without disease the test correctly classifies
Predictive Values - Measure test performance in a
particular population, what proportion of people who
test positive/negative do/don’t have disease
What is sensitivity?
Proportion of people with the disease who test positive = a/(a+c)
What is specificity?
Proportion of people without the disease who test negative (d/b+d)
What is the trade-off between sensitivity and specificity?
A highly sensitive test may incorrectly class healthy individuals as positive (lower specificity), while a highly specific test might miss some cases of the disease (lower sensitivity).
