Preanalytical stability of zinc in whole blood serum and plasma tubes as measured by ICP-OES and colourimetry

BackgroundZinc deficiency is a global concern, particularly in low-income countries, but also among vulnerable groups in Western countries, such as children. Diagnosing mild or moderate zinc deficiency is however challenging because of nonspecific symptoms and due to circulating zinc showing only subtle changes, requiring high accuracy in measurement. Challenges to accurate measurement include variations from choice of analytical instrument, analysis performance, and preanalytical factors such as choice of sample matrices and delayed blood sample processing. This study aimed to examine the stability of zinc in plasma and serum, measured by the recommended inductively coupled plasma optical emission spectrometry (ICP-OES) method and a direct colourimetric assay on the fully automated Roche Cobas c702 analyzer.MethodsA total of 245 whole blood samples were stored at room temperature for 0-8 h after blood sampling, then centrifuged for 10 min at 2000 g (serum) or 5 min at 2650 g (plasma), frozen at -20°C, and analysed, respectively, on ICP-OES and Cobas, the latter with the colourimetric kit from Sentinel diagnostics.ResultsSerum zinc concentrations measured on Cobas and ICP-OES showed no statistically significant change up to 6 h and never exceeded acceptable limits. Plasma zinc concentrations increased steadily over time, exceeding acceptable limits after 6 h. There were statistically significant differences between zinc measurements on ICP-OES and Cobas in both serum and plasma.ConclusionsZinc is stable for at least 8 h in serum and up to 6 h in plasma when measured by either Sentinel diagnostic colourimetric method on Cobas or ICP-OES.