New in Serum Osmolality: Zander's Formula

Serum Osmolality and OGAP... two of the formulae chemistry techs can do in their sleep. We all know the standard textbook Worthley et al equations;

Serum Osmolality (all in mmol/L):

2 Sodium + Urea + Glucose + 1.2 Blood Alcohol

Osmolar Gap  

Measured Osmolality - Calculated Osmolality

To make it easier for doctors or nurses without a lab handy, many websites/apps are even set up to do the math, with the reference ranges and criticals on the side. We've used the same formula for more than twenty-five years.

Last year, 2013, Zander's Formula was released as a "better, more useful calculation" for osmolality.

Zander's Osmolality (all in mmol/L): 

(Sodium + Urea + Glucose + Potassium + Bicarbonate + Lactate +6.5)x0.985

How is it better? Not only does it show a much better correlation between calculated and measured, it also has the closest 95% limits of agreement, when compared to 36 other possible formulae, (including the Worthley.) 

This new formula also only uses analytes commonly found on a blood gas analyzer panel. This will enable, in the future, a calculated osmolar gap to be added on automatically to trauma panels, giving the clinician one more piece of the puzzle in differentiating metabolic acidosis.

This is important because with such improved accuracy of OGAP, we would reduce the necessity for follow up methanol or ethylene glycol testing on many of our trauma patients who present with elevated gaps due to renal failure/shock/lactic acidosis, thus speeding diagnosis. (Remember though, the measured osmolality MUST be done by freezing point method if looking for any types of alcohol, as vapour pressure methods don't detect volatile solutes such as ethanol/methanol)

That said, Zander's is built to be used broadly; it doesn't include ethanol contribution as a matter of course; you would have to add the 1.2(ALC) yourself, to rule out/confirm any gap being due to ethanol rather than methanol, mannitol, or any of the other usual suspects. Depending on your population and preferred use, it could do with some tweaking.