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25 Cards in this Set
- Front
- Back
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What is the definition of screening?
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test to identify individuals at sufficient risk of a specific disorder to warrant further investigation amongst persons who have not sought help
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What does A, B, C, D represent?
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A = true positive
B = false positive C = false negative D = true negative |
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What is the definition of specificity?
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Proportion of those without the disease that test negative
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What is the definition of sensitivity?
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Proportion of those with the disease that test positive
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What are the calculations for both of these definitions?
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Sensitivity = a/a+c
Specificity = d/d+b |
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How would you work out the prevalence of a disease?
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(a+c)/n
(n = total) |
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How would you work out the positive and negative predictive values?
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Positive = a/(a+b)
Negative = d/(d+c) |
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What happens to the positive predictive value if the prevalence decreases?
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It falls and a false positive dominates
Target high risk populations Negative predictive value increases |
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What must be determined in tests with a continuous marker?
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Threshold via a receiver operator characteristic curve
Plot true positive against false positive (1- specificity) to see the effect of the cut off and to determine the most appropriate cut off |
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What happens if the theshold if low?
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Many screen positive regardless of true state
Increased sensitivity and decreased specificity |
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What is used to summarise effectiveness of the threshold?
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AUC = 0-1 (high values are good)
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What is the post test probability given a positive result?
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Positive predictive value
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What is the probability of a positive result?
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Prob x sensitivity + (100 - prob) x (100 - specificity)
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What is the probability of disease given a positive result?
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Prob x sensitivity/ probability of a positive result
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How are these values used in multiple tests?
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Use results in series
Post test probability after the 1st = the pre- test probability for the 2nd |
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Why is this unlikely to work?
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As the tests are unlikely to be independent
you need the probability of the 2nd conditional on the different possible results for the 1st |
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Why is this difficult?
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problem finding suffiecient evidence to calculate all the detailed proabilities that are required
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What is vital?
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Systematic gathering and synthesis of evidence
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What must screening be?
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High accuracy
Available Acceptable Benefit Cost effective |
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What are the conditions of a screening test?
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Epidemiology understood
detectable risk factor/ disease marker Latent period/ early symptomatic stage |
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What must the test be?
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Simple
safe precise validated known test values and cut off level |
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Give 5 examples of screening?
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Cervical cancer
breast cancer abdominal aortic anneurysm antenatal Chronic (IHD/ diabetes) |
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What test is involved in the colerectal screening programme?
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Faecal occult test
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Is screening good in old age?
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Some evidence to suggest screening improves quality of life in terms of home care
Universal vs targeted Hospital vs primary care |
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What is the gold standard in screening?
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An invasive diagnostic test
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