Hyperglycemia causes pseudohyponatremia. The sodium is diluted by the osmotic movement of water from the intracellular to the extracellular compartment. I was taught the Katz conversion to correct the sodium, the sodium falls 1.6 mmol/L for every 100 the glucose is over 100. This comes from purely theoretical work and was published in a letter the NEJM in 1972.
In the 1999 Hillier et al published empiric data that showed the ratio to be 2.4 rather than 1.6.
This ratio is now has been adopted by Mass General Handbook of Internal Medicine.
When ever I encounter hypernatremia I use both formulas and I consistently found that the Hillier estimate overshot the final sodium. I wanted to do a study where I looked at hyperglycemia in dialysis patients and measured estimated final sodium versus actual final sodium to see which calculation worked better. It is a good study cohort because the lack of urine output guards against renal losses a potential source of error. Well, the study has been done. Tzamaloukas et al, published a nice study of hyperglycemic dialysis patients and found the ratio of change in glucose to change in sodium was to be 100:1.5, almost exactly the same as Katz’s calculation and consistant with my experience.
|The y-axis is the Katz estimate minus the actual final sodium. so a perfect estimate is zero. The x-axis is the average of the Katz and actual final sodium. The Katz conversion work well across a range of actual sodium levels.|
Update, Dr. Rondon, in a comment below and Martijn vd Hoogen on Twitter, believe that I made a mistake calling hyperglycemic hyponatremia, an example of Pseudohyponatremia. There is some precedent for this position but it is not universal. See this editorial by the American Association for Clinical Chemistry, or McGraw Hill Concise Dictionary of Modern Medicine