The influence of viscosity on dilution methods. Its problems in the determination of serum sodium.

Dilution with a roller-pump is a well known procedure in clinical chemistry. This method may be influenced by viscosity. A higher viscosity of the sample yields a lower sample output and consequently a larger dilution. If the substance to be determined in the diluted sample can be measured accurately, the influence of viscosity may be analysed easily. This was performed for serum sodium. It was shown that with extreme pathological viscosities (as are found e.g. in multiple myeloma and Waldenström's macroglobinemia) underestimations of more than 15% may occur. Especially for serum sodium, the physiological range of which is fairly small, this may provoke serious diagnostic and therapeutic problems. We propose to use viscosity independent dilution systems.