Comparison of dewpoint and freezing point osmometry.

Osmolality was measured on 32 clear serum samples and on 23 urines, with dewpoint and with freezing point osmometers. The observed regression equation was dewpoint = 0.96 freezing point +7.6, with a correlation mean for 32 clear sera analyzed by both methods was not significantly different. However, when 14 lipemic sera were analyzed, the dewpoint measurements were higher: X dewpoint = 291.6; X freezing point = 284.1. The between-run standard deviation observed for a normal serum pool yielded a coefficient of variation of 1.04 with the freezing point and 2.38 with the dewpoint instrument. Results indicate that the dewpoint device should be used with caution for lipemic serum samples.