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 presentDisease absent Test positive Test Negative

Disease presentDisease absent Test positiveTPFP Test NegativeFNTN

Disease presentDisease absent Test positive3 7 Test Negative189

Sensitivity= TP/TP+FN =3/3+1=3/4=0.75=75% SENSITIVITY The 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