Positive predictive value of diagnostic test

Title : RxPG TargetPG AIPG 2006 Book

Author : Dr.J.Mariano Anto Bruno Mascarenhas

Biostatistic 2 Questions (063 to 064)

AIPG 2006

Biostatistics

References

Park (obviously!!)

Methods in Biostatistics for Medical Students and Research Workers - 6^th Edition - B.K.Mahajan - Jaypee Publishers

Methods of Biostatistics - Bhaskar Rao - PARAS Publishers

High Yield Biostatistics

The Basic Concepts in Biostatistics (given in all books)

	Disease Status	Disease Status
Test Results	Diseased	Not Diseased	Total
Positive	a = true positive	b = false positive	a + b
Negative	c = false negative	d = true negative	c + d
Total	a + c	b + d	a + b + c + d

Formulae

Sensitivity = a/(a+c) x 100

Specificity = d/(b+d) x 100

Predictive Value of Positive Test = a/(a+b) x 100

Predictive Value of Negative Test = d/(c+d) x 100

Percentage of false positive = b/(b+d) x 100

Percentage of false negative = c/(a+c) x 100

063. In a village having population of 1000, we found patients with certain disease and the results of a new diagnostic test on that disease are given below.

	Disease
Test result	Present	Absent
+	180	400
-	20	400

Which of the following is the positive predictive value of diagnostic test in that population

1. 45

2. 31

3. 95

4. 50

Answer

2. 31

Reference

Park 18^th Edition Page 117

Quality

Thinker

Status

Repeat

QTDF

All Books

Discussion

		True Positive
Predictive Value of Positive Test	=	-------------------------------------	x100
		True Positive +False Positive

Explanation

		180
Predictive Value of Positive Test	=	------------	x100
		180 +400
		180
	=	--------	x100
		580
	=	31.034

Comments

Predictive value reflects the diagnostic power of the test. The 'predictive value of a positive test' indicates the probability that a patient with a positive test result has in fact; the disease in question. The more prevalent a disease, in a given population, the more accurate will be the predictive value of a positive screening test. the predictive value of a positive test falls as disease prevalence declines.

Ä In positive predictive test the denominator is: True positive + False positive.

Ä In negative predictive test, the denominator is: False negative + True negative

Tips

Be ready with all the formulae in Statistics. Often you may get numerical Problems