What is a clinical trial?

What is a clinical trial?

We previously considered Lind’s well-known clinical trial of treatments for scurvy. To give a more recent example of a clinical trial, Lissauer et al conducted a randomized trial of whether prophylactic antibiotics before miscarriage surgery would reduce pelvic infection among women and adolescents in four low-resource countries (Malawi, Pakistan, Tanzania, and Uganda).¹³ 

Patients were randomly assigned either oral antibiotics doxycycline and metronidazole or placebos. The primary outcome was pelvic infection within 14 days after surgery, using pragmatic broad criteria. Among 1,676 patients assigned antibiotic prophylaxis, 68 developed a pelvic infection according to the primary outcome definition, in comparison to 90 of 1,684 patients receiving placebo, giving a risk ratio of 0.77 (95% confidence interval 0.56 to 1.04; p=0.09). (See Table 3 for selected data from the trial.) The study indicates there was not a statistically significant association between use of prophylactic antibiotics and reduction of pelvic infection, when defined using a pragmatic definition that incorporated clinicians’ judgement.

Table 3. Data from a trial of prophylactic antibiotics for miscarriage surgery

A clinical trial may be broadly defined as “a research activity that involves the administration of a test regimen to humans to evaluate its efficacy and safety.”² This broad definition includes studies which do not follow more than one cohort of participants, such as some early phase clinical drug trials. However, a stricter definition of a clinical trial is “a prospective study comparing the effects and value of intervention(s) against a control in human beings.”¹⁴  By this stricter definition, the intervention(s) must not only be assigned prospectively but also an intervention group must be compared with a control group receiving no intervention or an alternative intervention. Clearly, Lind’s trial of treatments for scurvy and the more recent trial of antibiotic prophylaxis meet this stricter definition of a clinical trial.

Community intervention trials represent another type of experimental epidemiologic study.⁹ In this type of trial, an intervention such as water fluoridation to prevent tooth decay is assigned to one or more entire communities but not to one or more control communities. Interventions may also be assigned at a group level smaller than an entire community, such as a dietary intervention assigned by household or an environmental intervention assigned to by office or factory. Cluster randomized trials are trials where the intervention(s) are randomized at the group rather than individual level.⁹