[Pre-analysis variability: the effect of evaporation on the error of measurement, with special reference to sodium and potassium].

Among the general causes of preanalytical variability, one of the most important is represented by the error due to the sample evaporation. The measure of the sodium and potassium concentration on the serum pools and on a saline solution has allowed to define the different variables effect. On the ground of the experimental data the error by operative loss is: a) directly proportional to the evaporative time; b) directly proportional to the exposition temperature for values of relative humidity of 60-70%; c) remarkably influenced by the air-flow in the environment; d) widely conditioned by the container-geometry and particularly by the height/diameter ratio; e) the minimum evaporative loss happens with cups filled up to the 50-60%, with much higher values below the 30% of filling; f) the influence of the different cup composition (either glass or plastic) as well as the different medium (either serum or saline solution) is negligible. The actual analytical error can be kept into moderate and acceptable ranges (lower than 1% in the eight hours) if the environmental ventilation is controlled, and if it is chosen the proper shape of the cup, the right sample cup capacity and a suitable operative procedure. In particularly unfavourable conditions, on the contrary, the entity of the evaporative error can even exceed the 100% in the eight hours time of the daily work.