Wednesday EBDM Flashcards
Probability of disease is close to zero: test or treat?
Neither
What are diagnostic tests used for?
- establish a diagnosis in sick patient
- screen for disease in healthy patient
- provide prognosis
- monitor ongoing therapy
- the results are usually dichotomous
Probability of disease is moderate: test or treat?
test first
Probability of disease is closer to 100%? Test or treat?
treat first
What are two basic features of test?
- reliability and precision
- accuracy
What is accuracy?
- ability to hit the target
- truth
What is precision?
repeatability
How will you know if you are accurate/close to truth?
gold standard test
A sick person that is correctly diagnosed as sick?
true positive
A healthy person wrongly identified as sick?
false positive
A healthy person correctly identified as healthy?
true negative
A sick person wrongly identified as healthy?
false negative
P( A | B )
probability of A given that B is true
P ( Julie’s pregnancy test is + | Julie is pregnant )
TP, FP, TN, FN?
True positive
(probability that the test is positive if she really is pregnant
P ( Julie’s pregnancy test is - | Julie is pregnant )
TP, FP, TN, FN?
FN
P ( Julie’s pregnancy test is + | Julie is not pregnant )
TP, FP, TN, FN?
FP
P ( Julie’s pregnancy test is - | Julie is not pregnant )
TP, FP, TN, FN?
TN
Why should Diagnostic tests must be compared to Gold Standard tests?
- they define disease
however, they are often time consuming, dangerous, painful, costly
A gold standard test should…
be 100% sensitive and 100% specific
If a test is very sensitive (99%), what does this mean about how many FN are present?
very few FN
very seNsitive tests have very few false Negatives
N for negative
Sensitivity
probability that a test will be positive if a disease is really there
P ( T+ | D+)
sensitivity = number of TPs / (number of TPs + FNs)
SnOut
very Sensitive tests are used to rule Out disease
(100% sensitivity means that the test correctly recognizes all sick people, and correctly rules out disease in 100% of people who do not have it, zero FNs)
If you have a very specific test, what does this say about FP?
very specific test has very few FP
Specificity
- probability that test will be negative when there is no disease
- very good at picking out negatives
P( T- | D- ) - can only be calculated initially in samples of individuals who do not have the disease
- very sPecific tests have very few false Positives
- highly specific tests to rule in disease SpIn (specificity = ruled in)
SpIn
highly specific tests are used to rule in disease
ROC curve
Where is the sweet spot? Test performing at ideal?
Inflection point
Limitations
- we don’t know if our patients actually have the disease
- sensitivity and specificity don’t fit clinical reality well
- all we usually know is what the test result is and our estimate of how likely disease is
P( T+ | D+ )
P( T- | D- )
But if you want your test to be more sensitive, go further on ROC curve, what does this do to specificity?
increases False Positives, decreases specificity
How should the two by two table for true/false positive/negative be organized?
top: patients with disease | patients w/o disease
side: test is positive | test is negative
(upper left corner is +/+, lower right corner is -/-)
Positive Predictive Value
the proportion of people with a positive test who will actually have the disease
- depends on the prevalence of the disease in the population
PPV = TP/(TP+FP)
Negative Predictive Value
the proportion of people with a negative test who will not have the disease
- depends on the prevalence of the disease in the population
NPV = TN / (TN+FN)
Receiver Operating Characteristic (ROC) Curves
plot used to determine the best cut-off points for ‘positive’ and ‘negative’ results
- Sensitivity vs 1 - Specificity
- asterisk is where the curve would be PERFECT (gold standard); this is upper left of graph
What are three important things to note about Gold Standard Tests?
- they are the reference or criterion standard for diagnosis (this test defines the presence or absence of disease)
- all diagnostic tests should be evaluated in comparison to a Gold Standard reference test
- unfortunately, gold standard tests are usually expensive and difficult to obtain, sometimes dangerous or risky for patients
