Association between methodological characteristics and outcome in health technology assessments which included case series

Abstract

Case series constitute a weak form of evidence for effectiveness of health technologies. However, for a variety of reasons, such studies may be included in health technology assessments. There are no clear criteria for assessing the quality of case series. We carried out an empirical investigation of the association between outcome frequency and methodological characteristics in a sample of health technology assessments. Systematic reviews of functional endoscopic sinus surgery for nasal polyps, spinal cord stimulation for chronic back pain, and percutaneous transluminal coronary angioplasty and coronary artery bypass grafting for chronic stable angina were identified as containing more than forty case series. Data were extracted by one reviewer and checked by a second on population characteristics, outcomes, and the following methodological features: sample size, prospective/retrospective approach, consecutive recruitment, multi- or single-center organization, length of follow-up, independence of outcome measurement, and date of publication. Association between methodological features and outcome were explored in univariate and multivariate analyses using parametric and nonparametric tests and robust regression or analysis of variance/analysis of covariance, as appropriate. Included reviews contained between forty-two and seventy-six case series studies, involving 5 to 172,283 participants. Reporting of methodological features was poor and limited the analyses. In general, we found little evidence of any association between methodological characteristics and outcome. Sample size is used as an inclusion criterion in many reviews of'case series but was consistently shown to have no relationship to outcome in all analyses. A prospective approach was not associated with outcome. Insufficient data were available to explore consecutive recruitment. Mixed results were shown for length of follow-up, independence of outcome measurement, and publication date. We found little evidence to support the use of many of the factors included in tools used for quality assessment of case series. Importantly, we found no relationship between study size and outcome across the four examples studied. Isolated examples of a potentially important relationship between other methodological factors and outcome were shown, for example, blinding of outcome measurement, but these examples were not shown consistently across the small number of examples studied. Further research into the determinants of quality in case series studies is required to support health technology assessment.