Using the Evidence - Lecture Thirty-Five Flashcards
Screening
Screening
The widespread use of a simple test for a disease in an apparently healthy population
Screening programme
An organised system using a screening test among healthy people in the population to identify early cases of disease in order to improve outcomes
Screening test
Usually cheap, simple
Why try to detect disease early?
Limit the consequences of disease through early diagnosis and treatment
Screening
Aims to improve outcomes usually to reduce mortality
Long lead time =
Greater chance of detecting disease early
Critical Point One of Screening
Disease is not detectable
Critical Point Two of Screening
Screening may be of benefit
Critical Point Three of Screening
Usually diagnosed anyway - no benefit
Over-diagnosis
Someone might have an illness but not be the cause of their death
Intrinsic Test
Measures accuracy with sensitivity and specificity
Sensitivity
Proportion of people with the disease who test positive
Specificity
Proportion of people without the disease and test negative
Specificity limitations
Costs and risks of next step high
Sensitivity benefits
Detecting as many cases as possible important
Costs or risks of next step not too high
Sensitivity & Specificity
What proportion of people with or without disease the test correctly classifies
Predictive Values
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
Negative Predictive Value (NPV)
Proportion of people who test negative and don’t have the disease
Predictive Values
Influenced by disease prevalence in the population of interest, unlike sensitivity and specificity
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 intervention
Reassurance (true negatives)
Improved health of population
Potential for early detection and intervention
Reduced mortality and/or morbidity
Possibly less radical treatment required
Physical harms of screening programmes
From complications, invasive tests and/or treatments
Psychological harms of screening programmes
From anxiety from waiting, distress from invasive tests or procedures, knowing about serious diagnosis for longer, false negative or false positive results
Financial harms of screening programmes
To individual and health service
Harms of screening programmes
Lead time bias
Length bias
Lead time bias
Over-diagnosis and/or over-treatment- may increase morbidity without reducing mortality
Length bias
False positives - period of stress and uncertainty until diagnostic test
May diagnose a disease that would never have become apparent
