Meta-analyses: how to read a funnel plot

Researchers undertook a meta-analysis to evaluate the relative short term safety of carotid endarterectomy compared with carotid artery stenting.1 Randomised controlled trials were included if they compared carotid endarterectomy with carotid artery stenting in patients with carotid artery stenosis (with or without symptoms). In total, 10 trials were identified that reported short term outcomes. The primary endpoint was mortality or stroke within 30 days of the procedure.

The total overall periprocedural risk of mortality or stroke was lower for carotid endarterectomy than for carotid artery stenting (odds ratio 0.67, 95% confidence interval 0.47 to 0.95; P=0.025). A funnel plot was presented (figure⇓). Egger’s test gave a P value equal to 0.932.

Which of the following statements, if any, are true?

• a) The funnel plot is used to detect bias in trials included in the meta-analysis

• b) Publication bias will result in asymmetry of the funnel plot

• c) The result of Egger’s test indicates that bias existed in the trials included in the meta-analysis