The substance concentration of ionized calcium (cCa2+) in blood, plasma or serum preanalytically may be affected by pH changes of the sample, calcium binding by heparin, and dilution by the anticoagulant solution. pH changes in whole blood can be minimized by anaerobic sampling to avoid loss of CO2, by measuring as soon as possible or by storing the sample in iced water to avoid lactic acid formation. cCa2+ and pH should be determined simultaneously. Plasma or serum: if centrifuged in a closed tube and measured immediately the pH of the sample will be close to the original value. If delay has occurred between centrifugation and the measurement, causing substantial loss of CO2, equilibration of the sample with a gas mixture corresponding to PCO2 = 5.3 kPa prior to the measurement is recommended. Conversion of the measured values to cCa2+ (7.4) is only valid if the pH is in the range 7.2-7.6 Ca2+ binding by heparin can be minimized by using either of the following: a final concentration of sodium or lithium heparinate of 15 IU/ml blood or less, by use of calcium titrated heparin with a final concentration less than 50 IU/ml blood. Dilution effect can be avoided by use of dry heparin in capillaries or syringes. When heparin solutions are used errors due to dilution or calcium binding can be reduced using syringes with a heparin solution containing free calcium ions corresponding to the mean concentration of ionized calcium in normal plasma. Conditions for blood collection, storage, and transport to avoid preanalytical errors are described.