Mostafa, D., E. I. Elkhatat, P. Koppolu, M. Mahgoub, E. Dhaifullah, and A. H. Hassan. “Correlation of ABO Blood Groups and Rh Factor With The Severity of Generalized Chronic Periodontitis: Across Sectional Study in Riyadh, Saudi Arabia”. Open Access Macedonian Journal of Medical Sciences, vol. 7, no. 4, Feb. 2019, pp. 617-22, doi:10.3889/oamjms.2019.044.