Preanalytical procedures in the measurement of ionized calcium in serum and plasma.

To investigate the effects of sampling routines, ionized calcium (corrected for pH) was measured after sampling in different types of tubes and under fasting and postprandial conditions. The results are discussed with respect to analytical goals. Serum samples, collected in vacuum tubes and spun after coagulation, could be safely measured during 24 h. This time window was expanded to 48 hours by using gel-containing tubes. Properly prepared specimens withstand mail transportation. Different brands of gel-containing tubes gave different results, and these differences were clinically significant. An "American-type" breakfast causes an average decrease of corrected ionized calcium ion concentration of 0.04 mmol/l and an increased interindividual variation of results. Characterisation of reference intervals should include the "nutritional status" and the brand of vacuum tube used.