To assess clinical heterogeneity across two studies with respect to study population, interventions, and outcome measures, and to evaluate the influence of these sources of heterogeneity on the results of the studies.
The individual patient data were used from two randomised controlled trials investigating the effectiveness of conservative treatments in patients with tennis elbow in primary care. Patients were allocated at random to treatment with steroid injection, wait and see policy, non-steroidal anti-inflammatory drugs, placebo tablets, or physiotherapy. Outcome measures included severity of the main complaint, inconvenience of the elbow complaints, pain during the day, elbow disability, pain-free grip strength, and global improvement. All outcomes were assessed at 1, 6, and 12 months after randomisation.
The two study populations were similar with respect to age, sex, comorbid neck/shoulder complaints, and baseline scores for the severity of pain. However, significant differences were observed for employment status, duration of elbow complaints, dominant side affected, previous history of elbow complaints, and use of analgesics. Local injections differed between the two studies with respect to volume, number, and steroid preparation. However, after 1, 6, and 12 months, the treatment effects of steroid injections were very similar between the study populations.
Despite large differences in study population at baseline, the responses to steroid injections were remarkably similar. Also the responses to other conservative interventions and the placebo treatment were very consistent, suggesting a uniform course of a tennis elbow and a lack of influence of clinical heterogeneity.