Biostats: Screening Flashcards
What are the Levels of Prevention?
What is the difference between diagnostic and screening tests?
- Diagnostic tests are used for persons in whom disease is suspected, due to symptoms, physical findings, or laboratory findings.
- Screening tests are used to identify early signs of a disease in people with no signs or symptoms of the disease.
What is sensitivity?
probability the test will be positive in someone with the disease
What is Specificity?
What is predictive value positive? PVP
What is predictive value negative? PVN
How does prevalence affect sensitivity, specificity, PVP, PVN?
Sensitivity and specificity are independent. As prevalence increases, PVP will increase, and PVN decreases slightly.
What are post test probabilities?
POSTTEST (POSTERIOR) PROBABILITY
= the probability of the disease after a test result.
a. After a positive test result:
Posterior Probability of a Positive Test
= a/(a+b) = PVP
b. After a negative test result:
Posterior Probability of a Negative Test
= c/(c+d) = 1 - PVN
What is lead time and length bias?
a. Lead time bias = the tendency for screen-detected cases to APPEAR to live longer than clinically-detected cases because “lead time” has been added to their duration.
“Lead time” is the time by which a screening test advances the date of diagnosis from the usual symptomatic phase to an earlier pre-symptomatic phase.
b. Length bias (or length biased sampling) - The tendency for screen-detected cases to live longer than clinically-detected cases because they invariably include an over-representation of cases with slowly progressing disease.
Whenever cases vary in the lengths of their detectable preclinical phases, then a screening test will always detect a disproportionate number of cases of slowly progressing disease and miss rapidly progressing cases that are in a detectable preclinical phase only briefly.
