According to CLSI, cells should be removed or separated from serum or plasma as soon as possible after collection. If the cells remain in contact for extended periods of time with serum or plasma, the actual results may be significantly different.
It is important to keep stoppers on the tubes while in storage. Capillary blood specimens without stoppers will lose CO2 and pH will increase. Besides, a tube left uncovered can be exposed to contamination and evaporation, which in turn will lead to inaccurate test results.