Screening Flashcards
What is screening?
The widespread use of a simple test for a disease in an apparently healthy (asymptomatic) population
Aims to improve outcomes, reduce mortality
What is a screening programme?
An organised system using a screening test among asymptomatic people in the population to identify early cases 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
Not a diagnostic test
What is case finding?
Case finding includes examples like if you go into the doctor with a sprained ankle and they also decide to check you blood pressure and find that you have an elevated blood pressure
Examples of NZ national screening programmes
Antenatal screening for Down Syndrome and other conditions
BreastScreen
National Bowel Screening
National Cervical Screening
Why should we screen?
Need to ensure that implementing screening programme will be effective, appropriate and that the benefits outweigh the harms
Must understand history of disease and ensure that earlier prevention will lead to a better outcome
How do we determine if the disease is appropriate for screening?
Screening programmes are expensive and resource intensive, need to be sure we are screening for a disease with severe consequences. Need to consider mortality, morbidity and disability of the disease
Disease should be relatively common or reasonably severe
What is lead time?
Length of lead time differs via different conditions
Length of lead time determines screening, no point in screening after lead time as disease will become symptomatic
Longer lead time = greater chance of detecting disease earlier
Critical points in course of a disease
No point in screening at critical point 1 as disease will not be detectable
At critical point 3 disease is symptomatic, outcome may be irreversible hence disease consequences may not be able to prevented
Critical point 2 is best time to screen, may be the whole lead time or not, if it is after lead time diagnosing it may not extend their survival, only extends the length of time that they know they have the disease (potentially harm)
What types of bias can occur during screening?
Need to consider lead time bias: diagnose something early but cannot extend lifespan (increased length of time they know the disease, perceived survival time, but not the length of time they survival)
Over diagnosis - persons disease slow developing, person could die of something else before giving treatment, can lead to over treatment
How can we determine if the test is accurate?
Want to maximise the amount of people with true positives or negatives
Can assess accuracy of screening test by measuring sensitivity and specifity
What is sensitivity?
Sensitivity is the proportion of people with the disease who test positive, (a/a+c)
Tells us how good a test is at identity those with the disease
True positive/all those with disease (true positive + false negative)
What is specificity?
Specificity is the proportion of people without the disease who test negative (d/b+d)
Tells us how good a test is at identity those without the disease
True negative/all those without the disease (true negative + false positive)
When to use sensitivity or specificity?
To choose which to maximise consider consequences of missing cases (false negatives) vs. false alarms (false positives)
Sensitivity: We choose sensitivity if we have want to detect as many cases as possible, have plenty of capacity for diagnostic testing, diagnosis testing has little harm
Specificity: Diagnostic test expensive, long waiting list, potential adverse effects
What are predictive values?
Predictive values measure test performance in a particular population, what proportion of people who test positive or negative do or don’t have the disease
Predictive values are influenced by disease prevalence in the population
Specific to programme in a particular population
What is a positive predictive value?
Positive predictive values are the proportion of people who test positive and have the disease (a/a+b)
True positive/all people with a positive test
What is a negative predictive value?
Negative predictive values are the proportion of people who test negative and don’t have the disease (d/c+d)
True negative/all people with negative test
Do we have the resources for screening?
Manage participation (people need to be eligible), cost and accessibility, quality control and monitoring
Capacity to treat true positives
Many people over long period, cost vs. benefit (spending money may be more cost effective doing something else than screening)
Is the screening programme effective?
Need to ensure programme is effective, look at RCT of screening in other populations before doing our own screening, need to consider particular bias’s and find benefits ideally reduced mortality or mobilising
Benefits of screening programmes:
Benefits include potential for reduced mortality and/or morbidity, less radical treatment required if you were diagnosed early, reassurance for people who are true negatives and ideally improve health of population
What are harms of screening programmes?
Screening isn’t always a good idea, could harm people:
• If not implemented properly could increase health inequities from unequal participation or treatment
• Physical harms from complications of diagnostic tests or treatment, could occur from false positive test
• Psychological harm - stress from procedures, anxiety from receiving positive screening test which may be false
• Financial harm if private treatment if waiting list too long, costly health services
What are bias’s harms in screening?
May increase morbidity without reducing mortality = lead time bias
False positives - period of stress and uncertainty until diagnostic test
May diagnose a disease that would never have become apparent = length bias/over-diagnosis or treatment
Screening is biased towards detecting slowly developing disease that may never have require treatment (therefore better prognosis)
