Table 2.

Quality assessment and scoring of included case–control studies

Study: first author, year of publication	Subjects		Comparability				Outcome			(6)	Score
	(1)	(2)	Age	Sex	Ethnicity	Region	(3)	(4)	(5)
Ahmed, 19909	*							*	*		3
Ahmed, 199130	*							*			2
Niizeki, 199414	*	*						*	*		4
Lombardi, 199629	*							*	*		3
Carcassi, 199632	*	*			*	*		*			5
Delgado, 199712	*	*			*	*		*	*		6
Lee, 199817	*	*			*	*		*	*		6
Gonzalez-Escribano, 199816	*	*			*	*		*	*		6
Miyagawa, 19998	*	*			*	*		*			5
Lombardi, 199915	*	*			*	*		*	*		6
Loiseau, 200013	*	*				*		*			4
Zhou, 200310	*	*	*	*	*	*		*	*		8
Veldman, 200431	*	*									2
Geng, 200528	*	*	*	*	*	*		*	*		8
Sáenz-Cantele, 200733	*	*				*		*	*		5
Shams, 200934	*	*				*		*	*		5
Saha, 201027		*			*	*		*			4
Tunca, 201035	*	*		*		*		*	*		6

Asterisks indicate criteria that were satisfied. (1) Patient selection criteria, including specific definition of the diagnosis; (2) representative control group (unrelated and healthy individuals); (3) blinding during data collection and interpretation; (4) study results clearly presented (odds ratio, P-value); (5) reporting of negative results (lacking statistical significance); (6) the prevalence of each variant in the general population (Hardy–Weinberg law of genetic equilibrium).