title: A1C between 5.7 and 6.4% as a marker for identifying pre-diabetes, insulin sensitivity and secretion, and cardiovascular risk factors: the Insulin Resistance Atherosclerosis Study (IRAS).
abstract: OBJECTIVE
  A1C is an optional method for diagnosing diabetes and also for detecting individuals at increased risk of the disease. However, how A1C compares with fasting (FPG) and 2-h plasma glucose for detecting at-risk individuals is not well known.
  
  
  METHODS
  A 2-h glucose tolerance test, frequently sampled intravenous glucose tolerance test, and A1C were obtained at the follow-up examination in 855 participants in the Insulin Resistance Atherosclerosis Study (IRAS). For this report, 385 individuals were at increased risk of diabetes as defined by A1C between 5.7 and 6.4%, impaired glucose tolerance (IGT), and/or impaired fasting glucose (IFG).
  
  
  RESULTS
  IFG and IGT identified 69.1 and 59.5% of all individuals at increased risk of diabetes, respectively. A1C 5.7-6.4% detected 23.6% of all at-risk individuals, although more African Americans (31.4%) and Hispanics (35.2%) than non-Hispanic whites (9.9%). Relative to A1C, FPG was more strongly related to fasting insulin (r = 0.38 vs. 0.26; P < 0.01), acute insulin response (r = - 0.20 vs. - 0.09; P < 0.01), and waist circumference (r = 0.43 vs. 0.25; P < 0.001) by the Spearman correlation test. Similarly, 2-h plasma glucose was more strongly related to Si (r = - 0.40 vs. - 0.27; P < 0.01) and triglycerides (r = 0.30 vs. 0.08; P < 0.001).
  
  
  CONCLUSIONS
  A1C 5.7-6.4% is less sensitive for detecting at-risk individuals than IFG and IGT, particularly among non-Hispanic whites. Single determinations of FPG and 2-h plasma glucose seem to be more precise correlates of insulin resistance and secretion than A1C and, in general, better for other metabolic disorders.
authors.affiliation: Department of Medicine, University of Texas Health Science Center, San Antonio, Texas, USA. lorenzo@uthscsa.edu
authors.forename: Carlos
authors.initials: C
authors.lastname: Lorenzo
authors.forename: Lynne E
authors.initials: LE
authors.lastname: Wagenknecht
authors.forename: Anthony J G
authors.initials: AJ
authors.lastname: Hanley
authors.forename: Marian J
authors.initials: MJ
authors.lastname: Rewers
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Karter
authors.forename: Steven M
authors.initials: SM
authors.lastname: Haffner
mesh_terms: D000368:Aged; D002318:Cardiovascular Diseases / Q000175:diagnosis / Q000378:metabolism*; D005260:Female; D006442:Glycated Hemoglobin / Q000378:metabolism*; D006801:Humans; D007333:Insulin Resistance / Q000502:physiology*; D008297:Male; D008875:Middle Aged; D011236:Prediabetic State / Q000175:diagnosis* / Q000378:metabolism; D012307:Risk Factors
chemical_list: D006442:Glycated Hemoglobin