Diagnostic Tests/Sensitivity/Specificity/ROC Flashcards
1
Q
Screening Tests
A
- Medical tests or procedure performed on members (subjects)
- Defined, asymptomatic population or population subgroup
- Assesses likelihood of their members having a particular disease
2
Q
Screening tests DON’T…
A
- diagnose the illness
- Testing positive usually requires further evaluation with diagnostic tests/procedures
3
Q
Planning a Screening Program
A
- Disease/condition being screened should be a major medical problem
- Acceptable treatment should be available for individuals with disease
- Access to health care facilities/follow-up services should be available
- Disease should have a recognized course with early to late stages
- Tests/procedures should be accepted to general population
- Natural history of disease should be understood
- Policies and procedures should be determined to know who needs further testing/treatment
- Process should be simple enough to encourage large groups to participate
- Screening should be an ongoing activity
4
Q
“Perfect” Diagnostic Test
A
- All individuals without disease give one uniform value
- All individuals with disease give a different, uniform value
- All test results coincide with being diseased or disease free
5
Q
Normal/”Reference” Range
A
- Not all values are the same in disease free individuals
- Disease free individuals will have a range of values (Reference Range)
- “Normal” misrepresents the range - distribution may not be normal and the individuals could be in range and not be healthy
6
Q
Reproducibility/Reliability of Test
A
- Ability to produce consistent results when repeated under same conditions
- Re-testing requires operator/observer to remain the same and blinded to results of 1st test
7
Q
Kappa
A
- K
- Index of agreement
8
Q
Precision
A
- Agreement of repeated measurements
- Range of results +/- SD
9
Q
Accuracy of Test
A
- Agreement of test values with true value
- Accuracy requires reliability or precision
- Results can be reliable without being accurate
- (T-) + (T+)/All
10
Q
Validity of Test Types
A
- Content validity - test represents what is trying to be measured
- Criterion Validity - Test results agree with other results measuring the same parameter
- Construct Validity - Real biological property being measured to explain why results vary among individuals
11
Q
Diagnostic Test Results
A
- Most often think or patients having or not having disease
- Nearly all disease processes are continuous
- Disease severity and number of symptoms tends to increase over time
- More difficult to diagnose early stages of disease while late stages are more distinct
12
Q
“The Gold Standard”
A
- Test whose results determine disease status
- Assumed to be 100% accurate/true
- Many diseases lack a test like this
- New tests are always compared to gold standard
- New drugs always compared to placebo control or current drug of choice (active control)
- Accuracy of new tests are always less or equal to gold standard
- New tests can outperform gold standards and become new gold standards
13
Q
Steps Determining Diagnostic Performance
A
- Choose gold standard test
- Perform gold standard test on a full spectrum of subjects
- Test all subjects with new diagnostic test
- Compare results of both tests in a 2x2 table
- Calculate the proportions of accurate and inaccurate results of the new test
14
Q
Prevalence + Diagnostics
A
- If proportion with disease is high, easy to diagnose a disease positive
- If proportion with disease is low, easy to diagnose a disease negative
- Surrogates for positive/negative disease diagnosis determine if patients will respond to drug therapy and likely drug therapy failures
15
Q
Reporting Prevalence
A
- Percentage
- Number of cases per a total number of people
16
Q
Types of Prevalence
A
- Point prevalence - proportion of people with disease at a specific point in time
- Period prevalence - proportion of people with the disease during a time period
- Lifetime prevalence - proportion of people with the disease at some point in their lifetime
17
Q
Prevalence
A
- Incidence rate X duration of disease
- Incidence - number of new cases in population over a period of time
- High incidence with long duration will have high prevalence
18
Q
Data Analysis
A
- Analysis of the comparison of the diagnostic test to gold standard requires independence of each test
- If both tests both misclassify the same patient, falsely high agreement with an overestimate of sensitivity and specificity
- If they independently misclassify the same patient, underestimate sensitivity/specificity
19
Q
Sensitivity
A
- Likelihood of a test reflecting that someone has a disease that we know they have
- (T+)/(T+) + (F-)
20
Q
Specificity
A
- Likelihood that a test will show that a patient doesn’t have a disease that they don’t have
- (T-)/(F+) + (T-)
21
Q
PPV
A
- Likelihood that a positive test result reflects a patient actually having a disease
- (T+)/(T+) + (F+)
22
Q
NPV
A
- Likelihood that a negative test result actually reflects a patient being disease free
- (T-)/(F-) + (T-)
23
Q
Predictive Values
A
- Specificity and sensitivity have no meaning to individual patients
- Patients’ true disease status is unknown at time of testing
24
Q
Prevalence + Predictive Values
A
- As prevalence goes down so does the PPV
- As prevalence goes down, NPV rises
25
ROC Curve
- Receiver Operating Curve
- Position of the "cut point" between normal and abnormal test results determine the test's sensitivity/specificity
- Cut point usually determined by the reference interval
- Sometimes altered to enhance either sensitivity or specificity
26
ROC Curve Meaning
- Shows tradeoff between sensitivity and specificity
- ROC curve following left-hand border and then top border shows greater test accuracy
- The closer it approaches a 45 degree diagonal, lower the test accurcy
- At the 45 degree angle, test is no better than a coin toss
27
Pretest Probability
-Probability of disease before application of the results of a physical finding
28
Post-test Probability
-Probability of a condition being present after a diagnostic test
