Haematocrit influences insulin concentration measurements in dried blood spots.

Introduction: A major barrier to conducting pharmacokinetic studies in neonates is the relatively large blood volume required by most assays. The use of dried blood spots (DBS) has potential to enable the use of smaller volumes and simplify sample processing and handling.

Aim: The aim of this study is to determine the effect of haematocrit on insulin concentrations from DBS.

Methods: DBS of varying haematocrit (0.25-0.65) were prepared at three insulin plasma concentrations (10, 25, and 50 mU/L). DBS were analysed for insulin using the method developed by Butter et al. (2001). DBS and paired plasma samples were obtained from neonates at Dunedin Hospital NICU.

Results: Insulin chemiluminescence responses were significantly lower at higher haematocrit values (p < 0.05). All results showed high variability (CV% = 9-61%). Calculated whole blood concentrations were plotted against chemiluminescence and an exponential function [Formula: see text] fitted. Plasma insulin concentrations from neonatal DBS were typically higher than paired plasma samples.

Conclusions: Haematocrit has a significant effect on insulin measurement by chemiluminescence when using DBS. Plasma insulin concentration could be determined when haematocrit was known. DBS insulin concentrations higher than plasma indicate that insulin may be present in red blood cells. However, measuring plasma insulin concentrations with DBS in neonates is not ideal due to high variability.