Claims of equivalence in medical research: are they supported by the evidence?

BACKGROUND: Most clinical studies are done to show comparative superiority, but many reports now claim equivalence between the investigated entities. These assertions may not always be supported by the methods used and the results obtained. PURPOSE: To assess the justification and support for claims of clinical or therapeutic equivalence in medical journals. DATA SOURCES: A search of MEDLINE for articles published from 1992 through 1996. STUDY SELECTION: From 1209 citations that contained the word equivalence in the title or abstract or contained the Medical Subject Heading therapeutic equivalency, we excluded 1121 studies reporting nonoriginal research, purely laboratory or other nonhuman research, and studies in which equivalence was not the main claim. The remaining 88 eligible papers were evaluated for five methodologic attributes. DATA SYNTHESIS: Only 45 (51%) of the 88 reports were specifically aimed at studying equivalence; the others either tried to show superiority or did not state a research aim. The quantitative distinctions regarded as "equivalent" ranged from 0% to 21% for direct increments and from 0% to 76% for proportionate differences. An equivalence boundary was set and confirmed with an appropriate statistical test in only 23% of reports. In 67% of reports, equivalence was declared after a failed test for comparative superiority, and in 10%, the claim of equivalence was not statistically evaluated. The sample size needed to confirm results had been calculated in advance for only 33% of reports. Sample size was 20 patients per group or fewer in 25% of reports. CONCLUSIONS: Many studies of clinical equivalence do not set boundaries for equivalence. Claims of "difference" or "similarity" are often made not by thoughtful examination of the data but by tests of statistical significance that are often misapplied or accompanied by inadequate sample sizes. These methodologic flaws can lead to false claims, inconsistencies, and harm to patients.     

Selective decontamination of the digestive tract: cumulating evidence, at last?

Selective decontamination of the digestive tract (SDD), an infection-control strategy designed to prevent nosocomial pneumonia in mechanically ventilated patients, has been implemented in numerous studies for more than 2 decades, but its role remains controversial. Sentinel studies in the 1960s and 1970s identified a link between colonization of the upper respiratory tract and subsequent increased risk of developing nosocomial pneumonia in critically ill patients. Studies in the 1980s found that prophylaxis with topical and systemic antibiotics to decontamination of the upper respiratory tract and gastrointestinal tract (particularly depleting gram-negative aerobic bacteria) was associated with lower rates of infections. However, impact on survival was not substantiated. However, several recent studies (including randomized trials and meta-analyses) suggest that SDD may improve survival in selected cohorts of critically ill patients in intensive care units (ICUs). Because liberal use of SDD (or any antimicrobial prophylactic strategy) may lead to escalating antimicrobial resistance, the risk of resistance varies according to local pathogens and resistance patterns. This review describes the development of the SDD concept, discusses recently published trials, and develops points for discussion and research. Additional studies are required to further define appropriate indications and limitations of this preventative strategy.