Stability of plasma electrolytes in Barricor and PST II tubes under different storage conditions

Sample stability can be influenced by many different factors; evaporation and leakage from residual cells are the most relevant factors for electrolytes. During the analytical phase, samples are usually kept uncapped at room temperature. Once samples are processed, they are usually stored sealed and refrigerated. Long turnaround time and the possibility of "add-on test" need consideration for electrolyte stability. The aim of our study is to examine short-term electrolyte stability in this two-common laboratory working conditions in two different lithium heparin plasma tubes (Barricor and PST II, Becton Dickinson).

In 39 plasma samples from voluntary subjects we measured sodium (Na⁺), potassium (K⁺) and chloride (Cl^-) at 6 time points since centrifugation (0h, 3h, 6h, 9h, 12h and 15h). Maximum allowable bias (clinically significant change) was based in SEQC (Sociedad Espańola de Química Clínica) recommendations; 1% for Cl^-, 0.6% for Na⁺ and 4% for K⁺.

In open room temperature tubes, clinically significant changes appeared in Na⁺ and Cl^- after 3 hours and in K⁺ after 9 hours in both types of tubes. In refrigerated sealed tubes, all the analytes were clinically stable up to 12 hours in both kinds of plasma tubes. We observed a statistically significant progressive increase in K⁺ levels, which was less pronounced in Barricor tubes.

Stability of electrolytes is compromised after 3 hours in open tubes and after 12 hours in sealed tubes.