The fundamental tenet of screening for any disease is that finding the disease before symptoms develop enables detection at a less advanced stage, and that initiating treatment at that time will reduce the adverse effects of the disease.43
The World Health Organization (WHO) outlined the principles of effective screening in 1968 and they are as true today as they were then. In their simplest form, the WHO screening guidelines encompass five key points, each of which applies to breast cancer and mammography:
The disease being screened is serious and prevalent,
The test is sensitive and specific,
The test is well tolerated,
The test is inexpensive, and
The test changes therapy or outcome.
The implications of these principles are that the disease must be prevalent enough within the population or subpopulation being screened to warrant testing individuals who show no signs or symptoms of the diseases. The value of different screening schedules and/or different technologies are likely to be different for different subpopulations, and the determination of optimal strategies requires careful study. Methodological challenges and studies currently under way are discussed in Chapter 6.
The value of a screening tool is determined by the relationship between the nature of the disease being screened and the performance characteristics of the screening tool. Sensitivity and specificity are the two most commonly cited measures of a screening or diagnostic test, but the more informative measure is the positive predictive value, which incorporates both sensitivity and specificity (see Box 2-1a and 2-1b).
Sensitivity refers to the proportion of true-positive results; this is calculated by dividing the number of breast cancer cases that were detected by the total number of breast cancer cases in the population tested, which equals the sum of those that were detected plus those that were missed. Estimates of the sensitivity of screening mammography from different studies range from 83 to 95 percent.72
Specificity refers to the proportion of true-negative results, or tests that correctly indicate that a woman does not have breast cancer among screened women without breast cancer. Mammography specificities generally fall in the range of 90 to 98 percent.72 In other words, the risk of a false-positive mammogram is about 1 in 10. Two studies suggest that among women who receive annual mammograms for 10 years, half will have at least one suspi-