What's the level of evidence behind the IDSA practice guidelines?

Unfortunately not as good as you might think:

Results In the 41 analyzed guidelines, 4218 individual recommendations were found and tabulated. Fourteen percent of the recommendations were classified as level I, 31% as level II, and 55% as level III evidence. Among class A recommendations (good evidence for support), 23% were level I (greater than or equal to 1 randomized controlled trial) and 37% were based on expert opinion only (level III). Updated guidelines expanded the absolute number of individual recommendations substantially. However, few were due to a sizable increase in level I evidence; most additional recommendations had level II and III evidence.

But EBM doesn't insist on RCTs. It only calls for the best available evidence. In a field like ID RCTs are often unavailable and may not be ethical.

Conclusions More than half of the current recommendations of the IDSA are based on level III evidence only. Until more data from well-designed controlled clinical trials become available, physicians should remain cautious when using current guidelines as the sole source guiding patient care decisions.

Via Hospital Medicine Quick Hits.