The fall of the serum anion gap.
The serum anion gap was an entry on the electrolyte region of NephMadness. It won it’s opening round over urine anion gap and advanced to the Sweet 16 by beating hypertonic saline but failed to win the electrolyte region when it fell to Bicarbonate in CKD.
CJASN did a recent review of the anion gap and they were frank with its limitations:
Some interesting notes about the anion gap: though the serum potassium was included in the original derivation of the calculation, none of the major U.S. textbooks include it. The upper limit of normal anion gap from the 8 sources included in the paper is much higher than I teach. I use 12 and after the ASN Board Review Class, I remember feeling that was too high.
The article then discusses the fact that ion selective electrodes are more sensitive for chloride so they detect higher chloride concentrations so that average and pathologic anion gaps are lower.
The article discusses the importance of albumin in the normal anion gap. As albumin falls, either the limit for a pathologically elevated anion gap needs to fall or the calculated anion gap needs to be adjusted upwards. The article recommends the latter. The anion gap should rise 2.5 for ever 1 g/dL the albumin falls below normal (presumably 4 g/dL). The authors recommend that albumin adjustments be incorporated into laboratory reporting so clinicians do not need to worry about this.
The meat of the article is contained in table 3 where the authors review 5 studies that looked at the sensitivity of an increased anion gap for lactic acidosis. It’s not a pretty picture.
|Both James and Jean Luc start with normal anion gaps, though James is at the upper limit and Jean Luc is at the lower limit of normal.|
|Both patients develop equivalent and significant lactic acidosis but only James develops an anion gap|