At 6 months , mean HbA1c levels in the periodontal therapy group increased 0.17 % ( SD , 1.0 ) , compared with 0.11 % ( SD , 1.0 ) in the control group , with no significant difference between groups based on a linear regression model adjusting for clinical site ( mean difference , -0.05 % [ 95 % CI , -0.23 % to 0.12 % ] ; P = .55 ) .