epidemiology of screening Flashcards
what is a screening test ?
a test that is done among apparently well people to identify those at an increased risk of a disease or disorder
what are the features of a screening test ?
screening :
- done to apparently asymptomatic individuals
- applied to a group of individuals
- results are based on one criteria
- results are not conclusive and less accurate
what are the features of a diagnostic test ?
- done to those with suggestive signs and symptoms
- applied to a single person
- results are based on evaluation of a number of symptoms, signs and investigations
- results are conclusive
what are the uses of screening programmes ?
case detection
control of diseases
research purposes
what are the types of screening programmes ?
- mass screenning
2. high risk or selective screening
what is the difference between mass screening and high risk or selective screening ?
- mass screening involves the whole population
2. high risk screening involves a selective group of the population
what are the four indicies of screening test performance ?
sensitivity
specificity
positive predictive value
negative predictive value
what does validity mean ?
how good is the screening test compared with the confirmatory diagnostic test
and how well does it measure what its supposed to measure
all the indicies of a screening test put together is the validity
what is a gold standard test ?
a test has been generally accepted, at that moment, as the standard and the best method for definitive diagnosis of that disease
what does sensitivity describe ?
the ability to identify correctly those who truly have the disease
how is sensitivity calculated ?
true positives
___________
all diseases
all diseased is also = true positives + false negatives
what is the formula for sensitivity ?
a
______. x 100
a + c
what is specificity ?
the ability of the test to identify correctly those who dont have the disease
how is specificity calculated ?
true disease negatives
___________________
all non diseased
what is the formulae for specificity ?
d
_____x 100
b + d
how is the overall validity calculated ?
true positives + true negatives
_______________________
grand total
which values are used to address the probability of the disease ?
positive predicted value
negative predicted value
what does the positive predictive value describe ?
the number of people with positive test results who actually have the disease
how is the positive predicted value calculated ?
true disease positives \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ all positives ( by the test)
what is the predictive value a reflection off ?
the diagnostic power
what are situations that require a test with high sensitivity ?
transfusion-donation safety
cancer
how can the rate of false positive results be minimized ?
using a test with high specificity
what is the relationship between positive and negative predicted values with the prevalence ?
PPV is directly proportional to the prevalence
NPV is inversely proportional to the prevalence
what does reliability in a test mean ?
does the test produce the same result when repeated applications are made on the same individual
what factors may affect the reliability of a test ?
intrasubject
intraobserver
interobserver
what is the formula for the positive predictive value ?
a
_______ x 100
a + b
how is the negative predictive value calculated ?
True disease Negative (TN)
___________________
All Negatives (by the test)
what is the formula for the negative predictive value ?
d
____ x 100
c + d
