Human plasma C-peptide immunoreactivity: Its correlation with immunoreactive insulin in diabetes, and chronic liver and renal diseases.

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Abstract
The correlation between plasma C-peptide immunoreactivity (CPR) and immunoreactive insulin (IRI) was investigated during the oral glucose tolerance test in 20 normals, 127 diabetics, and 39 non-diabetics with chronic liver or renal disorders. When all subjects were included, the increment of CPR 30 minutes after glucose load (4CPR) correlated well with that of IRI (ΔIRI)(r=0.66, p<0.001), but the return of CPR towards the basal level was delayed as compared with IRI. The positive correlation was also observed between the sum of 6 IRI and that of6CPR values during the glucose tolerance test in diabetics and controls (r=0.53, p<0.001).ΔCPR/ΔBS (30min.) was also well correlated with ΔIRI/ΔBS (30min.), and was specifically low in diabetics. Insulin-treated maturity-onset diabetics showed low but considerable CPR responses while no CPR responses were observed in insulin-treated juvenile diabetics. In each plasma sample, CPR always exceeded IRI on the molar basis. At fasting CPR/IRI ratio was 15.6±1.7 (mean±SE) in normals and 14.9±1.3-16.9±1.0 in diabetics. In chronic liver diseases IRI response was augmented while CPR response was not different from that of controls, and the molar ratio of CPR/IRI was significantly low (9.5±1.1). On the contrary, it exceeded that of normals in chronic renal diseases (35.7±14.9). It is concluded that, first, the plasma CPR response appears to be a valuable indicator of pancreatic B-cell function, and second, it is, nevertheless, modified in chronic liver or renal disorders.