CH AP TER 16 Principles and Practice of Secondary Prevention 20 Disease Occult disease Overt disease Death Rapidly progressive onset (age 60) Lead time D (age 45) (age 50 Slowly progressive (6 cases Figure 16-2 Lead-time bias Overestimation of survival duration among detected cases(relative to those detected by signs and symptoms) detected diagnosis. However, this simply reflects earlier diagnosis, because e overall survival time of the patient is unchanged. From Black WC, Welch HG: Advances in diagnostic imaging and overestimates of disease prevalence and the benefits of therapy, N Eng/ Med 328: 1237-1243, 1993) Test Time o E Time of disease onset DX Time when disease is clinically obvious without testing occurred, so that the period from diagnosis to death is increased. However, the additional lead time(increased time during which diagnosis is known)may not have changed the Figure 16-3 Overestimation of survival duration among screening. natural history of the disease or extended the longevity of detected cases, This is caused by the relative excess of slowly life. This lead-time bias tends to operate in screening fo cases, which are disproportionately identified by screening because the anders, no matter how aggressive the tumors( Fig. 16-2 portional to the le ngth of time during which they are detectable(and thereby inversely proportional to the rate of Length bias occurs when the full spectrum of a particular progression. In these 12 patients, 2 of 6 rapidly progressive cases are tumor, such as prostate cancer, includes cancers that range detected, whereas 4 of 6 slowly progressive cases are detected.(From from very aggressive to very slow-growing Individuals witl Black wC, Welch HG: Advances in diagnostic imaging and overestimates of slow-growing tumors live longer than individuals with the disease prevalence and the benefits of therapy, N Engl Med 328 ive tumors, so they are more likely to be discovered 1237-1243, 1993) by screening. Screening programs often select for the less aggressive, slower-growing tumors, and these patients are ikely to survive longer after detection, regardless of the treatment given(Fig. 16-3) women had an average of four mammograms and five clin Selection, lead-time, and length biases apply to both case cal breast examinations during this decade, and almost one ding and to community screening. Given the potential third had at least one false-positive examination. Recom- oblems in showing the true effectiveness of screening, mending frequent repeat examinations carries a significant great care must be exercised to ensure a community screen- burden of cost and anxiety to rule out disease in individuals ing program is worthwhile with false-positive examinations F. Repetition of Screening Programs G. Simultaneous Screening for Multiple Diseases There are pitfalls carefully considering the details of (Multiphasic So creen repeat screening efforts. This rticularly true if an initial ajor scre reening effort is considered a great success, and Multiphasic s involve screening for a enthusiasm may lead the organizers to repeat the screening variety of diseases in the same individual at one point in too soon(e. g, I year later). Unless the population screened time. Some investigators have argued that multiphasic the second time is very different from the one screened the screening makes community efforts more efficient. when a first time, a screening effort repeated after a short interval is sample of blood is drawn, for example, it is easy to perform ikely to be disappointing. This is because the initial screen a variety of tests, using modern, automated labora ing would have detected prevalent cases(cases accumulated over many years), whereas the repeated screening would However, the yield of multiphasic screening is doubtful detect only incident cases(new cases since the last screen- One problem is that multiphasic screening in an elderly ing), making the number of cases detected in the second population detects many diseases or abnormal conditions screening effort smaller. that have been found earlier and are already being treated, Again, the more screening tests done on an individual, the in which case funds are being used for unnecessary testing. more likely positive findings will occur, both true positive Another problem is that multiphasic screening results in a and false positive. If a woman begins annual breast cancer relatively high frequency of false-positive results, which screening at age 40, she would undergo 30 screening mam- requires many participants to return for more expensive nograms by age 70. One study followed 2400 women age 40 follow-up tests to 69 for a 10-year period to determine the number of mam For each disease-free person screened with a battery of nograms and clinical breast examinations done.' The independent tests(tests that measure different values), the