Quality assessment
The quality of the included studies was assessed by quality assessment
tools of the National Heart, Lung, and Blood Institute (NHLBI) (20). We
used a tool for observational cohort studies and another tool for a case
series study. Each tool composed of some questions to assess the risk of
bias and confounders. These questions were answered by “yes”, “no”,
“not applicable”, “cannot determine”, or “not reported” then each
study was given a score to guide the overall rating of the quality as
“good”, “fair”, or “poor”. We could not assess the publication
bias due to the small number of included studies according to Egger’s
funnel-plot-based methodologies (21).