6 Epidemiology Screening Flashcards
What is a screening test
a test which is used to detect disease and disease indicators in an apparently healthy population.
What is a diagnostic test
a test which is used in someone presenting with symptoms to establish a diagnosis
Sensitivity
proportion of people with the disease who test positive
true positive/have the disease
Specificity
proportion of people without the disease who test negative
true negative/do not have the disease
Positive predictive value
proportion of with a positive test who have the disease
true positive/total positive test
Negative predictive value
proportion of with a negative test who do not have the disease
true negative/total negative test
Specificity and sensitivity…
DO NOT change with a change in prevalence as they are features of the actual test not the disease
PPV and NPV…
DO CHANGE with a change in prevalence as they describe the disease.
Prevalence is low = less people are positive = low PPV
Prevalence is low = more people are negative = high NPV
Regarding accuracy what is wanted
a high NPV (close to 100%) to avoid false negatives
Describe lead time
extra time during which you know you have the disease if it is diagnosed by screening rather than by clinical presentation.
lead time bias: survival will look longer in screened individuals seemingly like those patients are doing better.
Survival time should not be used to evaluate screening
Describe length time
diagnosing disease that is less aggressive than disease that presents clinically.
length time bias: these cancers only able to be detected by screening. People would think that screening has helped the patient as the cancer hasn’t spread, however it is only that the cancers detected aren’t that harmful/rapid.
Describe over diagnosis
screening detects cancers that may not have even been detected clinically in their lifetime.
may go on to have surgery invasive treatment, could have been avoided if they weren’t overdiagnosed.
