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Diagnostic Test Characteristics
Professor Mobeen Iqbal
Shifa College of Medicine
Case
• At the local outpatient clinic of the department of gastroenterology, 100 consecutive patients are included in a study for a new non-invasive diagnostic test for detection of carcinoma of the colon. The new test was positive in 3 out of 4 patients with biopsy proven diagnosis of carcinoma. The test was also positive in 7 patients without biopsy proven carcinoma.
Disease present Disease absent
Test positive
Test Negative
Disease present Disease absent
Test positive TP FP
Test Negative FN TN
Disease present Disease absent
Test positive 3
7
Test Negative 1 89
• Sensitivity= TP/TP+FN
• =3/3+1=3/4=0.75=75%
SENSITIVITYThe proportion of people with the diagnosis (N=4) who are correctly identified (N=3)
• Specificity=TN/TN+FP
• =89/89+7=89/96=0.93=93%
The proportion of people without the diagnosis (N=96) who are correctly identified (N=89)
• PPV=TP/TP+FP
• =3/10=30%
• NPV=TN/TN+FN
• =89/90=99%
Person without the disease
Person with the disease
Person who tests positive
Person who tests negative
True positive on the test
False positive on the test
True negative on the test
False negative on the test
4 people with the disease, 3 are picked up by the test
Person without the disease
Person with the disease
Person who tests positive
Person who tests negative
True positive on the test
False positive on the test
True negative on the test
False negative on the test
test is positive for a further 7 people without disease
Person without the disease
Person with the disease
Person who tests positive
Person who tests negative
True positive on the test
False positive on the test
True negative on the test
False negative on the test
Person without the disease
Person with the disease
Person who tests positive
Person who tests negative
True positive on the test
False positive on the test
True negative on the test
False negative on the test
The remainder of the sample are negative on the test
Person without the disease
Person with the disease
Person who tests positive
Person who tests negative
True positive on the test
False positive on the test
True negative on the test
False negative on the test
Person without the disease
Person with the disease
Person who tests positive
Person who tests negative
True positive on the test
False positive on the test
True negative on the test
False negative on the test
SENSITIVITY
Person without the disease
Person with the disease
Person who tests positive
Person who tests negative
True positive on the test
False positive on the test
True negative on the test
False negative on the test
Person without the disease
Person with the disease
Person who tests positive
Person who tests negative
True positive on the test
False positive on the test
True negative on the test
False negative on the test
•SENSITIVITY is the proportion of people with the disease correctly identified by the test
•It measures the proportion of false NEGATIVES
SENSITIVITY
Person without the disease
Person with the disease
Person who tests positive
Person who tests negative
True positive on the test
False positive on the test
True negative on the test
False negative on the test
Person without the disease
Person with the disease
Person who tests positive
Person who tests negative
True positive on the test
False positive on the test
True negative on the test
False negative on the test
In this case, sensitivity is ¾ or 75%
SPECIFICITY
Person without the disease
Person with the disease
Person who tests positive
Person who tests negative
True positive on the test
False positive on the test
True negative on the test
False negative on the test
Person without the disease
Person with the disease
Person who tests positive
Person who tests negative
True positive on the test
False positive on the test
True negative on the test
False negative on the test
•SPECIFICITY is the proportion of people without the disease correctly identified by the test
•It measures the proportion of false POSITIVES
SPECIFICITY
Person without the disease
Person with the disease
Person who tests positive
Person who tests negative
True positive on the test
False positive on the test
True negative on the test
False negative on the test
Person without the disease
Person with the disease
Person who tests positive
Person who tests negative
True positive on the test
False positive on the test
True negative on the test
False negative on the test
In this case, specificity is (96-7)/96 or 93%
If someone is positive on the test, what are the
chances that he/she has the disease?
Person without the disease
Person with the disease
Person who tests positive
Person who tests negative
True positive on the test
False positive on the test
True negative on the test
False negative on the test
Person without the disease
Person with the disease
Person who tests positive
Person who tests negative
True positive on the test
False positive on the test
True negative on the test
False negative on the test
•Probability = 3/10 = 30%•This is the POSITIVE PREDICTIVE VALUE (the value of the test in predicting a positive result)
If someone is negative on the test, what are the chances that he/she does not have the
disease?
Person without the disease
Person with the disease
Person who tests positive
Person who tests negative
True positive on the test
False positive on the test
True negative on the test
False negative on the test
Person without the disease
Person with the disease
Person who tests positive
Person who tests negative
True positive on the test
False positive on the test
True negative on the test
False negative on the test
•Probability = 89/90 = 99%•This is the NEGATIVE PREDICTIVE VALUE (the value of the test in predicting a negative result)
SENSITIVITY, SPECIFICITY AND PREDICTIVE VALUES
• For sensitivity and specificity, the reference variable (‘denominator) is the DISEASE
• For predictive value, the reference variable (‘denominator’) is the TEST
acknowledgment
• Tom Sensky, Teaching EBM/EBMH
St Hugh’s College Oxford for some of the slides in the presentation
