Statistical analysis
Odds ratios (ORs) were calculated, and forest plots were constructed
using RevMan software (RevMan version 5.3 Windows; The Cochrane
Collaboration, Oxford, UK) by either fixed or random effects model based
on the level of heterogeneity. The level of heterogeneity in the forest
plot was measured by the Cochrane chi-square-based Q-test and was
considered significant if p -value<0.1 as described
elsewhere [44]. However, I2 statistic was used to
test the heterogeneity of included studies in which
I2<25%,
I2=25–50% and I2>50%
indicates low, moderate and high level of heterogeneity, respectively
[47]. A random effects model was applied to estimate ORs if
I2>50% and considered that the study had
high level of heterogeneity. In contrast, a fixed effects model was used
to estimate ORs if I2<50%. Sensitivity
analysis was carried out to assess the impact on any individual studies
on measured pooled risk. Publication biasness was detected by visual
inspection of the funnel plot where symmetrical distribution of the plot
indicated no publication bias [46]. All the calculatedP -values were considered statistically significant if the values
were<0.05.