EDTA improves stability of whole blood C-peptide and insulin to over 24 hours at room temperature

Abstract

Introduction: C-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.

Methods: We 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.

Results: C-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.

Conclusions: The 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.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1. Flow diagram detailing the sample collection protocol for the preservative and stability study over 24 hours.

At each time point the samples were centrifuged at 3000g for 10 minutes and the supernatant frozen at −80°C.

Figure 2. Stability of C-peptide and insulin in plasma and serum at room temperature and 4°C.

Blood taken from 3 non-diabetic volunteers assayed on the E170, Centaur and Immulite 2000 platforms. The samples were stored at room temperature (top row; figures A and B) and in the fridge at 4°C (bottom row; figures C and D) at 0.5, 1.0, 2.0, 6.0, 12 and 24 hours before freezing at −80°C. Data presented as mean percentage of baseline at each time point, with error bars representing the 95% confidence intervals. Red lines indicates samples collected K+-EDTA, blue lines indicate blood collected into serum gel, solid lines indicate whole blood and dashed lines indicate sample centrifuge at baseline.

Figure 3. Five day stability of C-peptide and insulin in whole blood stored on K+-EDTA and serum gel at room temperature and 4°C.

Blood taken from 5 non-diabetic volunteers assayed on the E170. The samples were stored at room temperature (top row; figures A and B) and in the fridge at 4°C (bottom row; figures C and D) at 0, 12, 24, 36, 48, 72, 96 and 120 hours before freezing at −80°C. Data presented as mean percentage of baseline at each time point, with error bars representing the 95% confidence intervals. Red lines indicate samples collected in K+-EDTA, blue lines indicate blood collected into serum gel.