IntroductionC-peptide and insulin measurements in blood provide useful information regarding endogenous insulin secretion. Conflicting evidence on sample stability and handling procedures continue to limit the widespread clinical use of these tests. We assessed the factors that altered the stability of insulin and C-peptide in blood.MethodsWe investigated the impact of preservative type, time to centrifugation, storage conditions and duration of storage on the stability of C-peptide and insulin on three different analytical platforms.ResultsC-peptide was stable for at least 24 hours at room temperature in both centrifuged and whole blood collected in K+-EDTA and serum gel tubes, with the exception of whole blood serum gel, which decreased to 78% of baseline at 24 hours, (p = 0.008). Insulin was stable at room temperature for 24 hours in both centrifuged and whole blood collected in K+-EDTA tubes. In contrast insulin levels decreased in serum gel tubes both centrifuged and whole blood (66% of baseline, p = 0.01 and 76% of baseline p = 0.01, by 24 hours respectively). C-peptide and insulin remained stable after 6 freeze-thaw cycles.ConclusionsThe stability of C-peptide and insulin in whole blood K+-EDTA tubes negates the need to conform to strict sample handling procedures for these assays, greatly increasing their clinical utility.

The method performs well and is now used in the routine analysis of sweat in this laboratory.

Background: Immunoassays for urinary albumin are often subject to the problem of antigen excess (the 'hook' effect) at high albumin concentrations. We developed an automated protocol to identify such samples based on urinary albumin to creatinine ratio (uACR) and urinary total protein (uTP) results. Methods: An automated flagging system was designed and written into the laboratory computer system to alert technical staff to samples potentially affected by the 'hook effect'. This flag was activated when there was a combination of an uTP of 2400 mg/L and an uACR of ,30 mg/mmol. Results: The potential rate of false-negative uACR results was approximately 0.17% in samples from primary care and diabetic clinic sources. Conclusions: Samples with falsely low uACR results were identified, allowing the vast majority of results to be authorized without intervention. The protocol prevented the reporting of false-negative uACR results which might impact on the management of patients.