—Thank you for your comments on C-peptide methodology, which are quite correct, regarding interference from insulin antibodies. The levels reported in the letter we published were obtained before precipitation of antibodies (nonpolyethylene glycol [PEG] values). However, on the day after the first determination, the assay was repeated after PEG precipitation of antibodies. This produced
a normal C-peptide response to glucose, given in the Table. The method was that of Kuzuya et al.1 All values of C-peptide obtained using PEG are comparable to values obtained using the C-peptide methodology in glucose tolerance testing from normal persons.