Biostatistics Questions

A new blood test to detect prostate cancer is evaluated in 300 male volunteers. A needle biopsy of the prostate gland is done on all men with serum prostate-specific antigen concentrations greater than 5 ng/mL (N<4). One hundred men undergo biopsy procedures; 90 are found to have prostate cancer, and five are found to have chronic prostatitis. Which of the following is necessary to calculate the sensitivity of this test?

• (A)  Incidence of chronic prostatitis in the general population
• (B)  Number of men with test results greater than 5 ng/mL and a normal biopsy specimen
• (C)  Prevalence of chronic prostatitis in the general population
• (D)  Prostate biopsies of men with test results equal to or below 5 ng/mL

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We know, 100 test positive, and that of those 90 have the disease (true positives), therefore, 10 do not have the disease but test positive (false positives). We also know there are 300 men in this study, therefore, 200 must test negative. However, we don’t know how many of these have the disease. Therefore, we need to know what the biopsy results were for the men who tested negative (i.e. PSA ≤ 5 ng/mL). Answer is D.

	Disease +	Disease –
Test +	TP = 90	FP = 10	100
Test –	FN = ?	TN = ?	200
	Sensitivity = TP/(TP+FN)	Specificity = TN/(TN+FP)	300