According to the Cochrane Collaboration- tests for funnel plot asymmetry should be used only when there are at least 10 studies included in the meta-analysis, because when there are fewer studies the power of the tests is too low to distinguish chance from real asymmetry.

For more- https://handbook-5-1.cochrane.org/chapter_10/10_4_3_1_recommendations_on_testing_for_funnel_plot_asymmetry.htm

Hope this is helpful.

Kaniz Afroz Tanni Thank you for your comprehensive explanation... Best regards...

It is good practice, as Kaniz Afroz Tanni has written, to have a minimum of 10 studies in the meta-analysis to perform tests for publication bias. However, it is still possible to detect publication bias having only 8 (or even 6) trials. Lifeng Lin et al. [1] found out that Tang’s regression test can be a better choice in such situations: "in particular, for small meta-analyses, (...) P values produced by Begg’s rank test and the trim-and-fill method were generally larger than those produced by regression tests. For example, among the meta-analyses containing 5 studies, most P values produced by Begg’s rank test and all P values produced by the trim-and-fill method were greater than 0.05, while the regression tests implied substantial publication bias with P values much less than 0.01 in some meta-analyses". Tang et al. used at least 6 studies for studying publication bias ("The arbitrary cutoff used here is based on the minimum number of six trials adopted in Egger et al’s paper in which the funnel plot method and the associated significance test are described in more detail") [2].

References:

[1] Article Empirical Comparison of Publication Bias Tests in Meta-Analysis

[2] Article Misleading funnel plot for detection of bias in meta-analysis