Table 2.

Bias assessment according to the Newcastle–Ottawa Quality Assessment Scale criteria.

Country	Study ID	Selection				Comparability	Outcome				Quality score
		Representativeness of exposed cohort	Selection of the non-exposed cohort from the same source as the exposed	Ascertainment of exposure	The outcome of interest was not presented at the start of the study	Comparability of cohorts	Assessment of outcome	Follow-up period long enough for an outcome to occur (median more than six months)	Adequacy of follow-up	Statistical test
Patients with thalassemia
Saudi Arabia	AlHamdan et al. (2007) [41]	Yes	Unclear	Yes (all screened individuals informed about the procedure)	Yes	Unclear	By clinicians	Unclear	Unclear	Unclear	Fair
	Alsaeed et al. (2011) [39]	Yes	Unclear	Yes (written informed consent obtained from all subjects)	Yes	Unclear	By clinicians	Unclear	Unclear	Available	Fair
	Alsaeed et al. (2018) [42]	Yes	Unclear	Yes (the data obtained in a deidentified format to protect the personal information of subjects)	Yes	Unclear	By clinicians	Unclear	Unclear	Available	Fair
	Ankra-badu et al. (2001) [43]	Yes	Unclear	Unclear	Yes	Unclear	By clinicians	Unclear		Yes	Unclear
	El-Hazmi et al. (1982) [44]	Yes	Unclear	Unclear	Yes	Unclear	By clinicians	Unclear		Yes	Unclear
	Memish et al. (2011) [45]	Yes	Unclear	Unclear	Yes	Unclear	By clinicians	Unclear	Unclear	Available	Fair
	Nasserullah et al. (2003) [46]	No	Unclear	Unclear	Yes	Unclear	By clinicians	Unclear	Unclear	Not available	Poor
	Ganeshaguru et al. (1987) [32]	No	Unclear	Unclear	Yes	Unclear	By clinicians	Unclear	Not available	Not available	Unclear
United Arab Emirates (UAE)	Barakat-Haddad et al. (2013) [36]	Yes	Unclear	Yes (the UAE ministry of education confirmed this study)	Yes	Unclear	Self-reported	Unclear	Unclear	Available	Good
	Denic et al. (2013) [37]	Yes	Unclear	Yes (written consent obtained)	Yes	Unclear	By clinicians	Unclear	Unclear	Available	Fair
	El-Kalla et al. (1998) [38]	Yes	Unclear	Yes (written consent obtained)	Yes	Unclear	By clinicians	Unclear	Unclear	Not available	Fair
Kingdom of Bahrain	Al-arrayed et al. (2003) [34]	Yes	Unclear	Yes (81% of parents' consent on screening)	Yes	Unclear	By clinicians	Unclear	Unclear	Unclear	Fair
Kuwait	Adekile et al. (1996) [35]	Yes	Unclear	Yes (written consent was obtained from the parents)	Yes	Unclear	By clinicians	Yes	Yes	Unclear	Good
Patients with β-thalassemia and sickle cell disease
Saudi Arabia	Pembrey et al. (1980) [30]	Yes	Unclear	Yes	Yes	Unclear	By clinicians	Unclear	Unclear	Not available	Fair
	Perrine et al. (1981) [31]	Yes	Unclear	Unclear	Yes	Unclear	By clinicians	Unclear	Unclear	Available	Fair
	El-Hazmi et al. (1985) [40]	Yes	Unclear	Unclear	Yes	Unclear	By clinicians	Unclear	Unclear	Not available	Poor
Bahrain	Rajab et al. (2006) [33]	Yes	Yes	Unclear	Yes	Yes	By clinicians	Unclear	Unclear	Available	Fair
Qatar	Fawzi et al. (2003) [47]	No	Unclear	Unclear	Yes	Yes	By clinicians	Yes	Yes	Not available	Fair