Question

A study is conducted of a doctor's ability to diagnose streptococcal throat infections in 160 patients visiting an emergency department (casualty department) in a community hospital. The doctor's clinical impressions are compared to the results of group A streptococcal throat cultures (Test). 40 of the patients had positive throat cultures, out of which the doctor has 30 streptococcal throat infection diagnoses. 120 patients had negative cultures, and the doctor diagnosed 40 of these as having streptococcal throat infection. Set up a 2 by 2 Table and do the following:

a.   Calculate the specificity of the doctor’s clinical judgment
b.   Calculate the sensitivity of the doctor’s clinical judgment
c.   Calculate the predictive value for positive test (PVP) of the doctor’s clinical judgment
d.   Calculate the predictive value for negative test (PVN) of the doctor’s clinical judgment

Answer 1

Solution: The 2 by 2 table for the above given question is as follows.

	Throat	infection	diagnosed
	Yes	No	Total
Positive culture	30(TP)	10(FP)	40
Negative culture	40(FN)	80(TN)	120
Total	70	90	160

Where TP= True positive, FP=False positive, FN=False negative and TN=True negative

a)

From the above table we calculate Specificity=80/(80+10)=0.89= 89% specific

b)

So Sensitivity=30/(30+40)=0.43=43%

c) PVP= Number of true positives/(Number of true positives+ Number of false positives)

= 30/(30+10)

=0.75=75%

d) PVN=Number of true negatives/(Number of true negatives+Number of false negatives)

=80/(80+40)

=0.67=67%