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Unlabelled: The determination of electrolytes in the clinical laboratories is carried out in samples of serum or plasma in situations that tubes remain open during a lingering time., Objective: To estimate the agreement and the interchangeability in the determination of electrolytes for three types of tubes (serum with separator, plasma with and without separator) in samples that remain at room temperature and open up to two hours., Material and Methods: Samples of 36 voluntary patients were selected determining the levels of sodium, potassium, chlorine, calcium, magnesium and phosphate in the three tubes on time 0 (basal), 60 and 120 minutes. The agreement was estimated by means of the intraclass correlation coefficient (ICC) and the interchangeability of results by means of lineal regression., Results: Agreement among the samples was very high with ICC higher than 0.9 in all basal determinations except for the potassium for which the presence of a constant systematic error was detected between serum and two tubes of plasma. The stability of the parameters is good in all the samples., Conclusions: It is possible the use of different types of tubes in the determination of electrolytes under the working conditions of the clinical laboratories. The determination of potassium needs to be corrected before the exchange of results obtained with serum and plasma. The stability in the two first hours is good in all the cases.