PASS Screening Flashcards
Two types of errors in screening programmes
False positive
False negative
What do false negatives provide?
Inappropriate reassurance
Delays treatment
What do false positives do?
Adds necessary stress and anxiety
Inconvenience
Costs wasted
Features to test validity
Sensitivity
Specificity
Positive predictive value
Negative predictive value
Sensitivity of test definition
Proportion of people with disease who test positive and are correctly identified
What is sensitivity of the test also known as?
Detection rate
Specificity of the test definition
Proportion of people without the disease who test negative and correctly identified as not having the disease
Positive predictive value definition
Probability that someone who tested positive actually has the disease
Negative predictive value definition
Probability that people who test negative actually don’t have the disease
Types of evaluation difficulty
Lead time bias
Length time bias
Selection bias
Lead time bias
- Early diagnosis falsely appears to prolong survival
- patient seems to have lived longer, only because they were diagnosed earlier
- lived same length of time but knew about the disease longer
Length time bias
- screening is better that identifying slow growing, unthreatening cases then aggressive, fast growing ones
- diseases detected are more likely to have never caused a problem
Selection bias
- worried by healthy individuals are more likely to take up screening programmes
- lower uptake in lower socioeconomic areas, ethnic minorities and socially excluded people
Ways of tackling selection bias
Building trust and rapport
Improving access to healthcare
Improving knowledge and awareness
How to calculate sensitivity
Correct positive/total positive cases
