This report is an addendum to the diagnostic criteria published in the 2006 WHO/IDF report “Definition and diagnosis of diabetes mellitus and intermediate hyperglycaemia” , and addresses the use of HbA1c in diagnosing diabetes mellitus. The WHO Consultation concluded that HbA1c can be used as a diagnostic test for diabetes, provided that stringent quality assurance tests are in place and assays are standardised to criteria aligned to the international reference values, and there are no conditions present which preclude its accurate measurement. An HbA1c of 6.5% is recommended as the cut point for diagnosing diabetes. A value less than 6.5% does not exclude diabetes diagnosed using glucose tests. The expert group concluded that there is currently insufficient evidence to make any formal recommendation on the interpretation of HbA1c levels below 6.5%.