Flashcards in Lecture 26: Screening; A Special Type Of Prevention Strategy Deck (47)
What is an example for primary prevention screening?
Screening women for alcohol intake to prevent breast cancer
What is an example of secondary prevention screening?
Breast cancer screening (detects early stage of disease and aims to prevent more serious disease
What is the screening test like?
Less expensive, less invasive (so effective on large population)
What's the diagnostic test like?
It's called a gold standard
Impractical for large populations
What happens after the gold standard diagnostic test?
Disease positive get the intervention or treatment
Disease negative get rescreened after a specific period
What happens to test negative after first screen?
They get rescreened after a specific period
What are the 4 screening criterion?
1. Suitable disease
2. Suitable test
3. Suitable treatment
4. Suitable screening time
Two features of a suitable disease?
1. An important public health problem
2. Knowledge of the natural history of the disease (or relationship of risk factors to the condition)
Is a suitable disease common or uncommon?
It can be either- if it's effective and easy, uncommon screenings can occur (eg babies screened for pku)
Why do we screen for uncommon disease too?
Because if we have an early detection and intervention we can get a better outcome
Why is knowledge of the disease important for screening?
So we know it's detectable early with a detectable risk factor/disease market and needs an increased duration of preclinical phase (after biological onset, before clinical diagnosis)
Examples of screening?
Breast cancer, cervical cancer, diabetes
What are the key aspects of a suitable test?
What is accuracy?
The ability of a test to indicate which individuals have the disease and which don't
Two measures of accuracy?
What two types of tests are used?
Gold standard test
What is the gold standard?
Is in effect the ideal diagnostic test (example: colonoscopy). Is expensive and invasive
What is a general screening test?
(A less expensive diagnostic test) eg faecal occult blood test (for colon cancer)
Describe the gate frame screening SQUARE
Top left: true positive
Bottom left: false negatives
Top right: false positives
Bottom right: true negatives
What is sensitivity?
The ability to identify correctly those who have the disease (a) from all individuals who have the disease (a+c)
The likelihood of a positive test in those with the disease
What is specificity?
The ability of the test to identify correctly those who do not have the disease (d) from all individuals who don't have the disease (b+d)
True positives/all with disease x100
"X% of those with the disease will test positive in the new test"
True negatives/all without the disease
= b/(b+d) x 100
"X% of those without the disease will have a negative result in the new test"
How do we evaluate test accuracy?
- the sensitivity is high if the proportion of true positives are high
- the specificity is high if the number of true negative are high
What are the fixed characteristics of a new test?
Sensitivity and specificity
What are the two predictive values?
Positive predictive value
Negative predictive value (NPV)
Why are sensitivity and specificity different between populations?
Because the prevalence of disease is different in different populations
So what is a predictive value?
How well a new screening test works in a specific population
What does the positive predictive value measure?
Out of everyone who tests positive (false and true), how many truly have the disease?
Or the probability of having disease if the test is positive