206 - Osmolality in serum and urine samples: calculated or measured?
Autor(s): V. Bianchi, P. Bidone, C. Arfini
Issue: RIMeL - IJLaM, Vol. 5, N. 3, 2009 (MAF Servizi srl ed.)
Page(s): 206-11
Summary
Background. In several clinical circumstances measuring osmolality both in serum and in urine samples is very important to make a right diagnosis. Usually in most laboratories mathematical formulas dealing with the concentration of sodium, glucose and urea are used, as their dosage is more easily determined on automated instruments. The reference method to detect osmolality is to measure the frozen point of the solution in comparison with water and the osmometer is the necessary instrument. The aim of this study is to evaluate if the calculated osmolality can substitute the direct measurement both in serum and in urine samples. Methods. Seven formulas among the principal ones in scientific literature have been chosen and used to calculate osmolality compared to the directly measured osmolality on 116 serum and 94 urine samples. The results have been compared through statistical methods, correlation coefficients and accordance have been evaluated. Results. With the exception of some formulas, both in serum and in urine samples the calculated osmolality is always lower than the measured osmolality. This can be explained by the presence in biological samples, in particular, in serum, of many analytes that contribute to the osmolality. Correlations between calculated vs measured osmolality are low in serum samples, whereas they are good in urine, independently from the mathematical formula used. Conclusions. Even if there are mathematical formulas in scientific literature, in order to correctly evaluate inpatients, the osmolality direct measurement should always be recommended. Key-words: osmolality, osmometer, cryoscopic measu- rement.