Evaluation of 36 formulas for calculating plasma osmolality.
OBJECTIVE
Measuring or calculating plasma osmolality is of interest in critical care medicine. Moreover, the osmolal gap (i.e. the difference between the measured and calculated osmolality) helps in the differentiation of metabolic acidosis. A variety of formulas for calculating osmolality have been published, most of them relying on sodium, urea and glucose. A novel formula developed by Zander has recently been published, which also takes into account the effects of potassium, chloride, lactate and bicarbonate on osmolality. We evaluate the previously published formulas including the novel formula by comparing calculated and measured osmolality.


METHODS
Arterial or venous blood samples from 41 outpatients and 195 acutely ill inpatients (total 236 subjects) were used to compare measured osmolality with calculated osmolality as obtained from 36 published formulas including the new formula. The performance of the formulas was statistically evaluated using the method of Bland and Altman.


RESULTS
Mean differences up to 35 mosmol/kg H(2)O were observed between measured and calculated osmolality using the previously published formulas. In contrast, the novel formula had a negligible mean difference of 0.5 mosmol/kg H(2)O. The novel formula also had the closest 95 % limits of agreement ranging from -6.5 to 7.5 mosmol/kg H(2)O.


CONCLUSIONS
Only 4 out of the 36 evaluated formulas gave mean differences between measured and calculated osmolality of less than 1 mosmol/kg H(2)O. Zander's novel formula showed excellent concordance with measured osmolality and facilitates a more precise diagnosis based on blood gas analysers. The new equation has the potential to replace separate measurements of osmolality in many cases.
Department of Respiratory and Critical Care Medicine, Otto Wagner Hospital, Sanatoriumstrasse 2, 1145 Vienna, Austria. andreas.fazekas@wienkav.at
Andreas S
AS
Fazekas
Georg-Christian
GC
Funk
Daniela S
DS
Klobassa
Horst
H
Rüther
Ingrid
I
Ziegler
Rolf
R
Zander
Hans-Jürgen
HJ
Semmelrock
D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D005260:Female; D006801:Humans; D008297:Male; D055641:Mathematical Concepts; D008875:Middle Aged; D009994:Osmolar Concentration; D010949:Plasma; D011446:Prospective Studies; D055815:Young Adult