Residual beta-cell function in children with type 1 diabetes: measurement and impact on glycemic control.

We studied residual beta-cell function in 84 children with IDDM to assess (1) relationship between basal and stimulated C-peptide concentrations; (2) reproducibility of testing (Group 1, n = 20); (3) the effect of exogenous insulin on test interpretation (Group II, n = 20); and (4) impact on metabolic control. Sustacal (a mixed liquid meal) was utilized as the test stimulus. In C-peptide positive subjects (n = 44) there was a strong correlation between basal and peak C-peptide concentrations (r = 0.88, p less than 0.001). In Group 1 subjects, Sustacal proved to be a highly reproducible test stimulus producing identical results on two tests 7-14 days apart. The results of the tests were not affected by the administration of subcutaneous regular insulin prior to the second test in Group II. Peak C-peptide correlated inversely with HbA1, insulin dose, and disease duration (r = -0.29, -0.40, and -0.33 respectively, p less than 0.05) and positively with age (r = 0.34, p less than 0.05). We conclude that Sustacal is a highly reproducible stimulus to residual beta-cell function in IDDM and is not affected by exogenous insulin. This residual insulin secretion has a small but significant effect on glycemic control.