Influence of Polyethylene Glycol (PEG) Extraction on the C-Peptide Determination in Sera from Insulin-Dependent Diabetic Patients with Circulating Insulin Antibodies^*)

Summary

To investigate whether the unexpectedly high C-peptide levels in some insulin-dependent diabetic (IDDM) patients are due to co-determination of proinsulin bound to circulating insulin antibodies, 36 randomly selected sera from IDDM patients were assayed for C-peptide immunoreactivity (CPR) after polyethylene glycol (PEG) extraction, preceding incubation with proinsulin binding antibodies (IAB + PEG) or without pretreatment of the sera. Recovery of proinsulin was checked by addition of 1 nmol/1 proinsulin to all sera. Recovery was found to be 101.5 ± 4.0%.

The mean values of concentrations were significantly lower (p < 0.001) after treatment with PEG and I AB + PEG compared to the untreated sera. There was also a significant difference (p < 0.05) between sera extracted with PEG alone or after IAB + PEG-treatment. However, no correlation (p > 0.1) was found to bound insulin (total minus free insulin) or to insulin binding capacity (IBC) of the sera.

If an antiserum is not available with very low cross-reactivity with proinsulin to determine human C-peptide then sera should not be extracted with PEG alone but after additional incubation with a proinsulin binding antiserum. In spite of the extraction in some cases unexplicably high C-peptide levels may still be expected.

Key words

Human C-peptide - Radioimmunoassay - PEG extraction - Type 1 Diabetes mellitus (IDDM) - Serum CPR concentrations