Prenatal Testing, Statistics, and Life-Altering Decisions

Much of my day is spent interpreting lab work.  Sometimes it is very easy but not always. Many families and health care professionals do not understand the concepts of sensitivity, specificity, positive predictive value and negative predictive value.  These values are affected greatly by the prevalence of the condition (or disease) that is being tested for in a specific population.

For many conditions, doctors prefer a highly sensitive test.  Tests that are highly sensitive will detect almost all of the individuals with the condition (or disease) being tested for and miss very few people (false-negative) with the condition. However, tests that are very sensitive often detect individuals who do not have the condition (false-positives). Therefore, when using tests with high sensitivity, more precise followup tests can determine conclusively if the condition (or disease) is present with much greater specificity.

A report from NBC news highlights how tests that are billed as “99 percent” accurate can be quite difficult to interpret and could lead to abortions of healthy fetuses.  Here’s the link: Sensitivity, Positive Predictive Value, and Prenatal Testing

Here’s an excerpt:

Positive results can be wrong 50 percent or more of the time…Noninvasive prenatal tests, or the “cell free DNA test,” are merely screening tests of placental DNA found in the mother’s blood…

The true likelihood that a positive test is positive depends on another calculation — the positive predictive value or PPV, which factors in other variables, such as a woman’s age and the prevalence of the disease in that population…

A woman over 35 where genetic disorders are more common — the likelihood of Trisomy 18 given a positive screening result is about 64 percent. For a younger woman, the PPV would be under 50 percent, according to the investigation.

Another example of understanding tests and statistics involves mammograms.  The relatively low reduction in averted cancer deaths related to mammograms has been discussed previously on this blog (see links below).  A good infographic and description is also available at NPR.  Here’s the link: What happens after your mammogram

 

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