title: The impact of insulin administration during the mixed meal tolerance test.
abstract: OBJECTIVE
  The mixed meal tolerance test is the gold standard measure of endogenous insulin secretion. Practical issues limit the routine clinical use of this test, including omitting insulin prior to the ingestion of a high-carbohydrate liquid mixed meal, which can result in marked hyperglycaemia. We aimed to assess whether insulin omission is necessary during the mixed meal tolerance test and whether fasting C-peptide was a practical alternative to the test.
  
  
  METHODS
  Ninety-one adults with insulin-treated diabetes (Type 1 n = 56, Type 2 n = 35) underwent two mixed meal tolerance tests; one standard without insulin and one with the patient's usual morning insulin.
  
  
  RESULTS
  The 90-min serum C-peptide was highly correlated in the standard mixed meal tolerance test and the test with insulin (r = 0.98, P < 0.0001). There was a 20% reduction in the peak C-peptide value when insulin was given {test with insulin [0.39 (0.01-1.16) vs. test without insulin 0.48 (0.01-1.36) nmol/l, P = 0.001]}, but the original serum C-peptide cut-off for significant endogenous insulin secretion (≥ 0.2 nmol/l) still correctly classified 90/91 patients (98% sensitivity/100% specificity). Fasting serum C-peptide was highly correlated to 90-min serum C-peptide during the test (r = 0.97, P < 0.0001). A fasting serum C-peptide ≥ 0.07 nmol/l was the optimal cut-off (100% sensitivity and 97% specificity) for significant endogenous insulin secretion (defined as 90-min stimulated serum C-peptide ≥ 0.2 nmol/l).
  
  
  CONCLUSIONS
  Insulin omission may not always be necessary during a mixed meal tolerance test and fasting serum C-peptide may offer a practical alternative in insulin-treated patients.
authors.affiliation: Peninsula NIHR Clinical Research Facility, Peninsula Medical School, University of Exeter, Exeter, UK. rachel.besser@pms.ac.uk
authors.forename: R E J
authors.initials: RE
authors.lastname: Besser
authors.forename: A G
authors.initials: AG
authors.lastname: Jones
authors.forename: T J
authors.initials: TJ
authors.lastname: McDonald
authors.forename: B M
authors.initials: BM
authors.lastname: Shields
authors.forename: B A
authors.initials: BA
authors.lastname: Knight
authors.forename: A T
authors.initials: AT
authors.lastname: Hattersley
mesh_terms: D000293:Adolescent; D000328:Adult; D017668:Age of Onset; D001786:Blood Glucose / Q000378:metabolism; D002096:C-Peptide / Q000097:blood; D003922:Diabetes Mellitus, Type 1 / Q000097:blood* / Q000188:drug therapy; D003924:Diabetes Mellitus, Type 2 / Q000097:blood* / Q000188:drug therapy; D004739:England; D005215:Fasting / Q000097:blood; D005260:Female; D005951:Glucose Tolerance Test / Q000379:methods*; D006442:Glycated Hemoglobin / Q000378:metabolism; D006801:Humans; D006943:Hyperglycemia / Q000097:blood* / Q000188:drug therapy / Q000517:prevention & control; D007004:Hypoglycemic Agents / Q000008:administration & dosage* / Q000097:blood; D007328:Insulin / Q000008:administration & dosage* / Q000097:blood* / Q000378:metabolism; D000078790:Insulin Secretion; D008297:Male; D062407:Meals; D008875:Middle Aged; D055815:Young Adult
chemical_list: D001786:Blood Glucose; D002096:C-Peptide; D006442:Glycated Hemoglobin; D007004:Hypoglycemic Agents; D007328:Insulin; C517652:hemoglobin A1c protein, human