I recently served as an expert witness in a malpractice case involving a family physician. The physician had seen a 13-year-old boy with a sore throat and ordered a rapid strep test, which was negative. A month later, the teenager presented with polyarthritis, weight loss and heart failure resulting from rheumatic fever. The patient required two heart valve replacements but is now a healthy 18-year-old, limited only by having to avoid contact sports because he takes warfarin.

The central point in the case was whether the physician digressed from the standard of care by not following up the negative rapid strep test with a throat culture. The opposing legal team produced infectious disease experts to testify that no competent laboratory in the country would fail to do so. They supported their claim with recommendations from the Red Book and numerous textbooks which stated that back-up cultures are necessary for a negative rapid strep test because of the test's low sensitivity. They also argued that CLIA and COLA regulations were violated because the package insert for the test included comments about following up negative tests with a culture.

The clinical issue is that rapid strep tests have very high specificity – in the range of 98 percent to 99 percent – so there are very few false positives. However, the sensitivity of most current rapid strep tests ranges from 90 percent to 95 percent, so there is a greater chance of false negatives. There have been few studies to show whether these patients with false-negative rapid strep tests are actually infected. One small study showed that about 40 percent of such patients do, in fact, have a strep infection, as indicated by a rise in strep antibody titers. Presumably the other 60 percent are non-infected strep carriers. This means that if 200 patients with sore throats presented to the office and the rapid strep test was the only method used, only about one patient would actually have strep and be missed because a follow-up culture was not done (assuming a rate of 20 percent strep).

In the 1990s, the infectious disease community spent a great deal of energy creating “expert” opinions recommending follow-up cultures for negative strep tests. At the same time, it became common practice in primary care to rely solely on the rapid strep test.

There are numerous valid reasons for not doing backup cultures. First, cultures are inherently flawed. They have always been difficult to do, and even in the best laboratories, they have at least an 8-percent false-negative rate comparing paired specimens. Additionally, the two- to three-day delay in test results is difficult to manage. Do you go ahead and treat patients with antibiotics for a few days even though they may not need the drugs, or do you make them wait for treatment?

At the same time, the national rate for rheumatic fever has dropped precipitously. With only one to two cases a year in a typical state, most family physicians have never even seen a patient with acute rheumatic fever.

The rapid strep test used by the family physician who was sued was, in fact, both highly sensitive (95 percent) and moderately complex compared with most rapid strep tests, which are CLIA-waived.

To figure out how to assess a 13-year-old boy with a sore throat, physicians have to weigh the inconvenience of cultures, the rarity of rheumatic fever and the risks of prescribing unnecessary antibiotics. While there are no national statistics, it is my observation that most family physicians rely on the rapid strep test and rarely do cultures, except when the clinical problem is not clearly solved in their minds. Although textbooks still support backing up a negative rapid strep result with a throat culture, the “standard of care” in practice has evolved so that back-up cultures are not necessary… and that is what I argued.

My examination on the stand lasted five hours. The next day, after only one hour of deliberation, the jury returned a 10-to-2 verdict in favor of the family physician. The real standard of care had won!