Published 21 October 2009, doi:10.1136/bmj.b4247
Cite this as: BMJ 2009;339:b4247

Endgames

Statistical Question

Screening tests III

Philip Sedgwick, senior lecturer in medical statistics

1 Centre for Medical and Healthcare Education, St George’s, University of London, Tooting, London SW17 0RE

p.sedgwick{at}sgul.ac.uk

The performance of digital mammography as a screening tool for breast cancer was investigated in premenopausal and perimenopausal women. Digital mammography returned a "screen positive" or "screen negative" result. Breast cancer status was diagnosed by biopsy (gold standard). Digital mammography was reported to have a sensitivity of 47%, specificity of 97%, and positive predictive value of 10%.1

Which of the following statements, if any, are true?

a) For every 100 women with diagnosed breast cancer, 47 will have had a "screen positive" result.
b) For every 100 women with a "screen positive" result, 47 will have diagnosed breast cancer.
c) For every 100 women without diagnosed breast cancer, 97 will have a "screen negative" result.
d) For every 100 women with a "screen positive" result, 10 will have diagnosed breast cancer.

Show Answers

Answers

Answer a is true. Of those women with diagnosed breast cancer, the percentage correctly identified by digital mammography with a "screen positive" result is termed the sensitivity of the screening test. Digital mammography was reported to have a sensitivity of 47%. Therefore, for every 100 women with diagnosed breast cancer, 47 will have a "screen positive" result that agrees with the biopsy diagnostic test.

Answer c is true. Of those women without a diagnosis of breast cancer, the percentage correctly identified by digital mammography with a "screen negative" result is termed the specificity of the screening test. Digital mammography was reported to have a specificity of 97%. Therefore, for every 100 women without a diagnosis of breast cancer, 97 will have had a "screen negative" result that agrees with the biopsy diagnostic test.

Answer d is true. Of those women with a "screen positive" result on digital mammography, the percentage of women who have diagnosed breast cancer is termed the positive predictive value of the screening test. Digital mammography was reported to have a positive predictive value of 10%. Therefore, for every 100 women with a "screen positive" result, 10 will have a diagnosis of breast cancer.

Answer b is not true. Of those women with a "screen positive" result on digital mammography, the percentage of women who have diagnosed breast cancer is termed the positive predictive value of the screening test. See answer d above.

Cite this as: BMJ 2009;339:b4247

Competing interests: None declared.

References

  1. Pisano ED, Gatsonis C, Hendrick E, Yaffe M, Baum JK, Acharyya S, et al; Digital Mammographic Imaging Screening Trial (DMIST) Investigators Group. Diagnostic performance of digital versus film mammography for breast-cancer screening. N Engl J Med 2005;353:1773-83.[Abstract/Free Full Text]

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