Epidemiology (III) Screening Basics Flashcards
What type of prevention is screening?
Secondary (early detection of disease)
What is the difference between a screening test and a diagnostic test?
Screening tests - detect early disease or risk factors for disease in large numbers of apparently healthy individuals. (identification in asymptomatic patients) Diagnostic tests - establish the presence (or absence) of disease as a basis for treatment decisions in symptomatic or screen-positive individuals (confirmatory test). (confirmation in symptomatic patients)
What part of the disease’s pathogenesis must be present for a screening to be useful?
A detectable preclinical phase
What are some considerations to make before developing a screening tool?
Is there a long, recognizable preclinical phase?
Can we improve the prognosis with earlier detection?
Is the test valid and reliable?
Is it sufficiently cheap and noninvasive?
How harmful can the test be?
What is external validity?
Generalizability of results to the general public
What is internal validity in screening?
The test measures what it is designed to measure
What is reliability in screening?
Consistency and replicability of the test
What are two measures of a screening test’s validity?
Sensitivity and specificity
What is screening sensitivity?
The ability of the test to correctly identify those with the disease
(the proportion of people with the disease who test positive)
A test with a high sensitivity has a low what?
(Hint: it is measured by 1 - sensitivity)
A low false-negative rate
For what type of test is a high sensitivity especially important?
Screening tests
Why are false negatives damaging?
They can lure diseased patients into a false sense of security and delay disease detection.
What is screening specificity?
The ability of the test to correctly identify those who do not have the disease.
(the proportion of people without the disease who test negative)
A test with a high specificity has a low what?
(Hint: it is measured as 1 - specificity)
A low false-positive rate
For what type of test is a high specificity especially important?
Confirmation tests
Why are false positives damaging?
They may cause unnecessary anxiety and panic (think of a diagnosis with HIV or metastatic pancreatic cancer)
How is sensitivity measured?
TP / (TP + FN)
(true positives divided by everyone that has the disease)
(how many of the diseased individuals did the test successfully identify?)
How is specificity measured?
TN / (TN + FP)
(true negatives divided by all the healthy individuals)
(how many of the healthy individuals did the test successfully identify?)
How can we identify which test should be the gold-standard diagnostic technique?
The test with the desired sensitivity and specificity is the gold-standard
What is important about a test’s cutoff point for a disease case definition?
It determines the properties of the disease (high sensitivity or high specificity)
What happens to specificity as sensitivity increases?
Specificity decreases (and vice versa)
What questions about false-negatives and false-positives should be asked when selecting how sensitive and specific a test will be?
What is the consequence of an undetected case (can we wait to test again later, or does the disease progress rapidly)?
What is the consequence of calling a disease-free person a positive (will there be invasive tests, stress, high costs, or time wasted)?
What does the ‘ROC’ in ROC curve stand for?
Receiver operating characteristic
How is an ROC curve formed?
By plotting the true positive rate (sensitivity) against the false positive rate (1 - specificity)
(basically, probability of detection vs. probability of a false alarm)
Where will a cutoff point do the least harm in the overlapping distributions of health and diseased individuals?
How can the best cutoff point be determined from an ROC curve?
The point nearest the upper lefthand corner
