Lessons From theMixedMeal Tolerance Test Use of 90-minute and fasting C-peptide in pediatric diabetes

OBJECTIVEdMixed-meal tolerance test (MMTT) area under the curve C-peptide (AUCCP) is the gold-standardmeasure of endogenous insulin secretion in type 1 diabetes but is intensive and invasive to perform. The 90-min MMTT-stimulated CP $0.2 nmol/L (90CP) is related to im-proved clinical outcomes, and CP $0.1 nmol/L is the equivalent fasting measure (FCP). We assessed whether 90CP or FCP are alternatives to a full MMTT. RESEARCHDESIGNANDMETHODSdCPwasmeasured during 1,334MMTTs in 421 type 1 diabetes patients aged,18 years at 3, 9, 18, 48, and 72 months duration. We assessed: 1) correlation between mean AUC CP and 90CP or FCP; 2) sensitivity and specificity of 90CP$0.2 nmol/L and FCP $ 0.1 nmol/L to detect peak CP$0.2 nmol/L and the equivalent AUC CP; and 3) how the time taken to reach the CP peak varied with age of diagnosis and diabetes duration. RESULTSdAUC CP was highly correlated to 90CP (rs = 0.96; P, 0.0001) and strongly correlated to FCP (rs = 0.84; P, 0.0001). AUC CP $23 nmol/L/150 min was the equivalent cutoff for peakCP$0.2 nmol/L (98 % sensitivity/97 % specificity). A 90CP$0.2 nmol/L correctly classified 96 % patients using AUC or peak CP, whereas FCP$0.1 nmol/L classified 83 and 85% patients, respectively. There was only a small difference seen between peak and 90CP (media