Sensitive/specificity/PPV/NPV/incidence/prevelance Flashcards
Sensitivity is also called
True positive rate
Fixed properties of a test
- Sensitivity
2. Specificity
Evaluation of a diagnostic test
Uses 2X2 table comparing test results with te actual presence of disease (vertical: TEST: + - / horizontal: disease: + -)
Sensitivity equation
TP/(TP+FN)=1-FN rate
Sensitivity - definition
Proportion of all people with disease who test positive or,
The probability that a test detects disease when disease is present
Value approaching 100% sensitivity is desirable for ruling …. disease
It indicates
OUT
A low false negative rate
High sensitivity test used for screening in disease with ….. prevalence
LOW
Specificity is also called
TN rate
Specificity
Proportion of people without disease who test negative
or,
The probability that a test indicates non disease when disease is absent
Value approaching 100% specificity is desirable for ruling …. disease
and indicates a
IN
Low false positive rate
High specificity test used for
Confirmation after a positive screening test
Positive predictive value
- Proportion positive test that are true postive
2. Probability that person actually has the disease given a positive test result
Positive predictive value equation
TP/(TP+FP)
PPV varies directly with
Prevalence or pretest probability
High pretest –> high PPV
Negative predictive value
- Proportion of negative test results that are true negative
- Probability that person actually is disease free given negative test result
Negative predictive value equation
TN/(FN+TN)
NPV varies inversely with
- Prevalence
- Pretest probability
High pretest probability –> low NPV
Prevalence
Number of existing cases/number of people at risk
At a point in time
Incidence rate
(Number of new cases in a specified time of period)/(population at risk during same time)
Prevalence directly reversible
- Incidence
2. Average disease dutation
Incidence and prevalence in common cold. Bigger?
Very similar because duration is short
Incidence and prevalence in chronic diseases. Bigger?
Prevalence. Long duration
Posspitive and negative predictive value vary depending on
Disease prevalence
Specificity equation
TN=TN/TN+FP
= 1-FP
High sensitivity test used for
SCREENING in diseases with LOW prevalence
town population is 7.500. In 2011, 200 residents diagnosed with RA. in 2012, 100 more residents are discovered by RA. incidence, prevalence?
incidence = 100/7300 prevelance = 300/7500
fixed properties of a test and properties that vary
fixed: specificity, sensitivity
vary: PPV, NPV (depending on disease prevalence)
association between PPV, NPV, sensitivity, specificity
high sensitivity –> high NPV
high specificity –> high PPV
