A comparison of calculated and estimated osmolarities of urine.

Osmolarity has been calculated from estimated cations and organic solutes in 200 samples of urine and compared with that found by direct osmometry. In three-fifths of the samples the calculated value differs from the found by less than 5%, but in some differences of over 30 m.osmoles/l. and percentage differences of over 20 are found. Calculated values are frequently too high, this overestimation being particularly marked in alkaline urines, and a change in reaction of the urine in one subject can alter the relationship of the calculated to the true osmolarity in the course of a few hours. The findings in a small series of samples where the principal anions are estimated suggest that the osmolar activities of the solutes are less in the more alkaline urines than in the more acid ones.