Anion gap
Background
A normal gap is 12 +/- 4 (i.e. 8 to 16). The anion gap is the difference between measured cations and measured anions in serum. This difference does not reflect a true disparity between positive and negative charges, given that serum actually is electrically neutral when all serum cations and anions are measured. Rather, the anion gap is a measurement artifact resulting from the fact that only certain cations and anions are routinely measured. Anion gap metabolic acidosis is secondary to the addition of endogenous or exogenous acid
- AG = Na - (Cl + HCO3)
- Normal Anion Gap = 12+/-4 (8-16)
- 12-20 mEq/L when including K+
Cutoffs for "normal" Anion Gap are laboratory and equipment specific. Newer technology and equipment have been shown to measure "low" AG in otherwise normal, healthy people.[1][2]
Elevated Anion Gap
Differential Diagnosis
MUDPILES
- M - Methanol, metformin
- U - Uremia
- D - DKA
- P - Paraldehyde, phenformin
- I - Iron, INH, ibuprofen (large ingestions)
- L - Lactic acidosis
- E - Ethylene glycol
- S - Salicylates
Also:
- Starvation/ETOH ketoacidosis
- Carbon Monoxide (CO), CN poisoning (increased lactate), colchicine
- Toluene
- Chronic acetaminophen use
KILR
- K – Ketoacidosis - (diabetic, Alcohol, Starvation)
- I – Ingestion - (Salicylates, Acetaminophen, Methanol, Ethylene glycol, CO, CN, Iron, INH)
- L – Lactic acidosis - (infection, hemorrhage, hypoperfusion, Alcohol, Metformin)
- R – Renal - (Uremia)
Low Anion Gap
Differential Diagnosis
- Lab error
- Decreased "unmeasured" anions
- Hypoalbuminemia
- Increased "unmeasured" cations
- Hyperkalemia
- Hypercalcemia
- Hypermagnesemia
- Lithium Toxicity
- Increased cationic paraprotein
- Multiple myeloma
- Polyclonal IgG gammopathy
- Pseudo
- Severe Hypernatremia (>170 meq/L)
- True conc of Na is underestimated
- Marked hyperlipidemia
- Leads to overestimation of plasma Cl conc)
- Bromide toxicity (e.g. for myasthenia gravis and some herbal medications)
- Assay mistakenly reads Br as Cl
- May have negative anion gap
- Elevated serum iodide
- Severe Hypernatremia (>170 meq/L)
- AG falls by 2.5 meq/L for every 1 g/dL reduction in albumin concentration
External Links
References
- Jurado RL, del Rio C, Nassar G, Navarette J, Pimentel JL Jr. "Low anion gap." South Med J. 1998;91(7):624
- Winter SD, Pearson JR, Gabow PA, Schultz AL, Lepoff RB. "The fall of the serum anion gap." Arch Intern Med. 1990;150(2):311
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