The review question and the inclusion criteria, with the exception of those for participants, were clear. The authors searched some relevant databases, but limited their search to studies reported in English. This might have increased the possibility that some relevant studies were not included in the review. The authors reported the use of methods designed to minimise bias and error in the extraction of data, but not in the selection of studies for the review. They stated that they did not assess study validity, which, even where exclusively trials described as RCTs are included, means it is difficult to assess the reliability of conclusions based on the trial data. The analysis methods were complex and the main aim of the paper was to perform a meta-regression to identify factors relating to the effects of TZD treatment on HbA1c. As the results of the individual trials were not presented in forest plots, it is difficult to assess whether the pooled results are reliable, although the authors did state that there was no evidence of statistical heterogeneity. The lack of a validity assessment and the reliance on a meta-regression, the results of which should be treated as exploratory, mean that the reliability of the conclusions cannot be established.
One author is an employee of, and holder of stock options in, Merck and Co.