The Problem of False Positives

Screening tests are assessed according to their sensitivity (the percentage of affected individuals detected) and their specificity (the percentage of unaffected individuals who are correctly excluded from further testing). The actual number of people who receive inaccurate initial screening results depends in large part on the frequency of the disease in the population. The more common the disease, the more likely it is that a person who receives a positive (abnormal) test result will actually be affected. (The rhetoric of screening and testing is confusing in that "positive" test results almost always mean that something is wrong.) As the disease becomes less frequent, the proportion of initial results that turn out to be "false positives" increases. Suppose a disease has an incidence of 1 in 10,000 and a population of 100,000 people is tested with a screening test that has a sensitivity of 90 percent (so that 9 out of 10 affected people will test positive) and a specificity of 99 percent (so that 99 out of 100 unaffected people will test negative). The results overall would be as follows:

Test positive Test negative

Affected 9 "true positive" 1 "false negative" Unaffected 999 "false positive" 98,991 "true negative"

Put another way, for every person who was truly affected (and tested positive), 100 people who did not have the disease would also (falsely) test positive. In addition, nine people who did have the disease would test negative. While most people who get false positive test results are ultimately reassured by further testing, some may continue to be worried. Affected children who are missed in these programs may face substantial delays in diagnosis if clinicians reason that the child could not have the disorder because it would have been identified in the newborn period.

The disorders sought in newborn screening programs typically are quite rare, usually having frequencies in the 1-in-5,000 to 1-in-15,000 range. Some of the diseases that are being added being to newborn screening panels are as rare as 1 in 100,000. Without denying the benefits that can come to affected children who are detected in these programs, it is important to acknowledge the possible harms that may befall the many children who inevitably receive falsely abnormal results. The newborn period is a particularly vulnerable time. Parents are just beginning to know and bond with their infants. Bad news, even if incorrect, can interfere with the formation of this central relationship and lead parents to view their new infants as medically fragile. One study revealed that almost 10 percent of parents whose infants received initial false-positive screening results for cystic fibrosis were still worried a year later that their children were affected or otherwise sickly.

Thus, the trend has been to increase the disorders for which newborns are screened, including some for which the benefits of early invention are unclear or may be absent, all the while causing a growing number of infants to receive false-positive test results, which will cause some of them harm.

Anxiety and Depression 101

Anxiety and Depression 101

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