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Tuesday, March 11, 2008

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 - 6th 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 18th 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

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