Inverted insulin to C-Peptide ratios in neonatal intensive care: is there something we don't know?

Insulin (I) and C-Peptide (C) are secreted in equimolar amounts, but insulin clears more rapidly. Typical Insulin to C-Peptide ratios (I/C) are therefore <0.05-0.2 (1/20-1/5). Except in relatively rare cases, I/C > 1.0 are interpreted as evidence of exogenous or factitious insulin dosing. However, recently published data from pre-term neonates with no insulin dosing or family history of diabetes show far higher ratios. How can these "inverted" ratios happen without factitious insulin poisoning?

Insulin binding antibodies explain the incidence and magnitude of "inverted" I/C ratios in this NICU cohort?

A secondary analysis integrating three independent evidence streams with mechanistic modelling study. Three "siloed" areas of neonatal research were integrated: (1) paired insulin and C-Peptide measurements from two NICU cohorts (227 and 75 matched samples) from infants not receiving insulin and with no immediate family history of diabetes or insulin use illustrates the incidence of unexpected I/C > 0.1-0.2; (2) Analysis of direct measurements of affinity, displacement and binding-site capacity across 5 independent studies constrain neonatal binding affinity and concentration; and (3) Analysis of binding kinetics of reversible insulin-binding reactions. Results of #2-3 are combined with a clinically validated neonatal metabolic model to assess whether they could account for observed I/C > 1.0 from #1.

For #1, over 80% of NICU infants had I/C > 0.1 and 40% had I/C > 1.0 consistent with circulating bound insulin measured by some immunoassays. #2, Independent affinity measurements, displacement studies, direct concentration measurements, and reconstruction from I/C data converged on a dominant high-affinity, low-nanomolar binding regime. #3, Model-based analysis of antibody binding reactions reproduced the observed I/C ratios and high apparent insulin concentrations without requiring extreme assumptions.

High I/C ratios are not uncommon in NICU neonates and can be explained by high-affinity insulin-binding activity rather than insulin administration. Multiple independent quantitative analyses converge on a common high-affinity, low-nanomolar binding regime which reproduces observed I/C ratios. Critical illness, such as sepsis, significantly increases the levels and likelihood of unexpected high I/C ratios.