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A single Roche Elecsys Immunoassay test for insulin and C-peptide is not a valid determinant of exogenous insulin administration in a preterm neonate due to several critical factors:

The Roche Elecsys assay has not been validated explicitly for preterm neonates, a population with unique metabolic profiles due to immaturity. No studies have confirmed its accuracy, sensitivity, or specificity in this group, and reference intervals are based on general or adult populations.

Preterm neonates exhibit significant variability in insulin and C-peptide levels due to factors like gestational age, insulin resistance, and transient hyperinsulinism.

Without continuous monitoring, short-term peaks or troughs in insulin or C-peptide are missed, as studies only sample over hours or days.  A single test cannot capture dynamic changes, risking misinterpretation of high insulin as exogenous when it could be a natural transient response.

The assay can be affected by interferences (e.g., cross-reactivity with proinsulin or split products, and matrix effects in neonatal blood, which are not accounted for in preterm-specific validations, potentially skewing results.

Even if a high insulin to low C-peptide ratio suggests exogenous insulin, preterm neonates can naturally have low C-peptide due to insulin deficiency or rapid clearance, alongside high insulin from resistance or stress. This overlap means the ratio is not a definitive indicator without context.

https://pmc.ncbi.nlm.nih.gov/articles/PMC6056384/

https://karger.com/neo/article/108/2/93/227641/Hyperglycaemic-Preterm-Babies-Have-Sex-Differences

https://bmjpaedsopen.bmj.com/content/8/1/e002470

https://bmjpaedsopen.bmj.com/content/1/1/e000160

https://labogids.sintmaria.be/sites/default/files/files/c-peptide_2018-08_v10.pdf

https://diagnostics.roche.com/global/en/products/lab/elecsys-c-peptide-cps-000460.html

https://www.labcorp.com/tests/010108/c-peptide

https://www.annlabmed.org/journal/view.html?doi=10.3343%2Falm.2018.38.6.530

https://wwwn.cdc.gov/nchs/data/nhanes/public/2009/labmethods/ins_f_met_insulin_elecsys_2010_immunoassay.pdf