Example 3: Rapid Strep Test II

Let us go back to the example of rapid strep test (RST) but change some of the parameters. Let us say that you are doing this testing in the middle of July. Of 1500 children tested, 1473 have a negative RST and 27 have a positive RST. In addition, a backup throat culture (gold standard) was done on all children. Of those with a negative RST, 1468 have a negative throat culture. In the group with a positive RST, 20 have a positive throat culture. Calculate the sensitivity and specificity of the RST. Using the table below that you used for the specificity and sensitivity in previous sections, calculate the positive predictive value and negative predictive value.

		Target Disorder (strep tonsillitis)
		Present	Absent
Diagnostic test result	Positive RST	20 = a	7 = b	27 (a+b)
	Negative RST	5 = c	1468 = d	1473 (c+d)
		25 (a+c)	1475 (b+d)	1500 (a+b+c+d)

Calculate the prevalence of strep in the first example (more like the typical situation in the spring) based on the information below:

		Target Disorder (strep tonsillitis)
		Present	Absent
Diagnostic test result	Positive RST	159	3	162
	Negative RST	36	1302	1338
		195	1305	1500