INTRODUCTION — Determination of the serum anion gap (AG) is primarily used in the differential diagnosis of metabolic acidosis [1-4]. (See "Approach to the adult with metabolic acidosis", section on 'Physiologic interpretation of the serum anion gap'.)
However, the serum AG can also become abnormal in other conditions, a finding that may be of diagnostic importance [1-5].
CALCULATION OF THE ANION GAP AND NORMAL VALUES — The serum AG is calculated from the serum sodium (Na), which is the principal cation in the blood, and chloride (Cl) and bicarbonate (HCO3), which are the principal anions (calculator 1):
Serum AG = Na - (Cl + HCO3)
Historically, the normal range for the AG has been 12±4 mEq/L. However, the normal range for each of the individual measurements varies depending upon the specific analyzers used. In addition, the normal range of the AG has decreased over the past several decades as new technology has been introduced, especially in laboratories using ion-selective electrodes (which report higher serum chloride concentrations) [6]. In several reports, for example, the normal range for the AG range was reported as 6±3 mEq/L [7,8]. Thus, it is best if each laboratory establishes its own "normal range" for the AG [3,9,10].
In addition, in some countries the serum potassium (K) is included in the AG measurement, resulting in a normal range that is approximately 4 mEq/L higher than the number calculated in the United States. If potassium is included in the formula, the AG calculation becomes:
Serum AG = (Na + K) - (Cl + HCO3)
The way that anions and cations in serum are interrelated is shown in the figure (figure 1). If all the anions and cations are included in the analysis (and their concentrations are reported in mEq/L), then the total anions = the total cations. The figure also shows that the AG could be alternatively determined with the following equation (figure 1):
Serum AG = All unmeasured anions - All unmeasured cations
For this analysis of the AG, unmeasured ions are defined as all serum ions other than sodium, chloride, or bicarbonate.
The alternative formulation demonstrates that an increased serum AG (defined by the difference between sodium and the sum of chloride and bicarbonate) can be due to an increase in unmeasured anions (anions other than chloride and bicarbonate) or a reduction in unmeasured cations (cations other than sodium). Conversely, a low or even negative AG will develop if there is a reduction in unmeasured anions or an increase in unmeasured cations [2,4,11].
Albumin molecules have many positive and negative charges due to amino acid side chains that can either release or bind hydrogen ions. The ratio of albumin's positive and negative charges changes with the pH of the solution. At pH 7.4, albumin carries approximately 16 more negative charges than positive charges (therefore each molecule has a net charge of -16). The major unmeasured anion in serum is albumin. Thus, the serum albumin must be measured and taken into account when calculating and interpreting the serum AG. If the albumin concentration is reduced, correct the AG as follows:
Corrected AG = AG + (2.5 x [4.5 - Albumin g/dL])
As an example, if the AG is calculated at 15 mEq/L and the albumin concentration is 2.0 g/dL, then the albumin-corrected AG is 21 mEq/L.
Other unmeasured anions in serum include phosphate, urate, and sulfate. Unmeasured cations include potassium (because potassium is usually not included in the AG calculation), ionized calcium, magnesium, and some abnormal, usually monoclonal, proteins.
HIGH SERUM ANION GAP — An elevated serum AG is most often due to an increase in unmeasured anions, and this is almost always caused by one of the organic metabolic acidoses (eg, lactic acidosis, ketoacidosis). (See "Approach to the adult with metabolic acidosis".)
Much less commonly, the AG elevation is due to marked hyperalbuminemia, hyperphosphatemia, or an anionic paraprotein (usually an immunoglobulin A [IgA] monoclonal immunoglobulin) [1-4,12-15]. Although a reduction in unmeasured cations could theoretically also raise the AG, this generally does not occur, because reductions of potassium, ionized calcium, or magnesium will have a relatively trivial quantitative impact on the AG [3,4].
A small elevation in serum AG also occurs in patients with metabolic alkalosis [15-18]. A study in dogs showed that chronic severe diuretic-induced metabolic alkalosis, which increased the serum bicarbonate to 40 mEq/L, increased the AG by 8 mEq/L [18]. Multiple factors increase the serum AG in metabolic alkalosis [18]:
●An alkaline plasma pH causes albumin molecules to release hydrogen ions (a protein-buffering effect), and this increases the net negative charge on each albumin molecule and thereby its contribution to the anion gap.
●Volume contraction, which accompanies the most common forms of metabolic alkalosis, will raise the albumin concentration and its negative charge contribution.
●Alkalemia increases the generation rate and accumulation of multiple organic acids, such as lactic acid. This mitigates the increase in bicarbonate and decreases the severity of alkalemia, while increasing the AG.
Lastly, any artifactual increase in sodium concentration or artifactual decrease in the chloride and/or bicarbonate concentration will cause the AG to artifactually increase [19-24]. An artifactual decrease in serum bicarbonate (pseudohypobicarbonatemia) can occur in the setting of hypertriglyceridemia when bicarbonate is measured spectrophotometrically (likely because triglycerides absorb light). This artifact does not occur when bicarbonate is measured using a blood gas analyzer [25-27].
LOW SERUM ANION GAP — The definition of a "low" serum AG must be determined by each laboratory but is generally a value less than 3 mEq/L [3,9]. Some etiologies of a low serum AG may also generate a negative serum AG (eg, high concentration monoclonal immunoglobulin G [IgG] gammopathy) (table 1). (See 'Negative serum anion gap' below.)
An important cause of a low or negative AG is random laboratory error [28-30]. Thus, when this is reported, the electrolyte measurements should be repeated to determine if the result is reproducible (table 1).
A reproducible low serum AG can occur as a result of [1,3-5,7,31]:
●A decreased concentration of "unmeasured anions." The most common etiology in this category is hypoalbuminemia [1,2,7,32].
●Severe hyperchloremic metabolic acidosis may reduce the AG because protons bind to albumin as the pH falls, and this reduces albumin's net negative charge. As an example, when chronic metabolic acidosis was produced in dogs by the infusion of hydrochloric acid (reducing their serum bicarbonate to 10 mEq/L), the AG fell by 5 mEq/L [16,17]. However, the calculated change in albumin charge only accounted for a portion of the change [17].
●An increased concentration of "unmeasured cations" [3,5,7,28]. Examples include hyperkalemia, hypercalcemia, and/or hypermagnesemia [1,3,33]. However, in order for these cation increases to reduce the anion gap, the increases in the unmeasured cation concentration cannot be accompanied by a proportional increase of an "unmeasured anion." As an example, if parenteral magnesium sulfate is administered, the increase of magnesium is approximately matched by an equivalent increase in serum sulfate (hence the AG does not change). However, if magnesium chloride is administered, then the chloride concentration increases without a change in sodium or bicarbonate, and the AG will fall.
Severe lithium intoxication may cause a low or negative serum AG in patients using lithium chloride or lithium bicarbonate. In such patients, a low AG may be a diagnostic clue that lithium concentrations are high [34,35].
●Monoclonal proteins, especially IgG proteins, are usually cationic. Thus, they represent unmeasured cations, and high concentrations will reduce the AG [3,15,34,36]. A reduced serum AG has also been reported in some patients with polyclonal IgG gammopathy [15,37,38].
●Chloride, bromide, and iodide are halide ions, and the last two can interfere with many chloride analytic devices and generate "pseudohyperchloremia," which thereby produces a low or negative AG [3,5,39].
●Increased concentration of other unusual serum anions such as salicylate or thiocyanate can also generate pseudohyperchloremia and reduce the anion gap into the abnormally low or negative range [5,40-45].
●Occasionally pseudohyperbicarbonatemia develops when the serum or plasma specimen contains high levels of the enzyme LDH (which is used in some HCO3 analyzers) [29]. This will reduce the anion gap.
NEGATIVE SERUM ANION GAP — When the serum AG has a negative value, it is important to rule out a sporadic (nonreproducible) laboratory error. If the laboratory results are reproducible, then the following possibilities exist:
●Pseudohyponatremia or an artifactual reduction of the sodium concentration can be generated by the following conditions (table 1):
•"Displacement errors" caused by high blood lipid and/or protein concentrations – Whenever the water phase of plasma or serum (normally about 93 percent of the total volume) is reduced and the assay methodology is dependent upon a “normal” water phase percentage, pseudohyponatremia will result. Marked hyperlipemia or hyperproteinemia will reduce the water phase percentage. The assay techniques affected by this artifact are flame photometry and indirect (meaning a dilution step is required) ion-specific electrode techniques [46-50]. (See "Causes of hyponatremia without hypotonicity (including pseudohyponatremia)".)
•Hyperviscosity – Whenever a quantitative volume of the sample must be diluted (usually with an automated aspiration device), hyperviscous serum or plasma may reduce the amount of aspirate and thereby artifactually reduce the results of any analyte subsequently measured. This generates pseudohyponatremia [44].
Note that measurement techniques that are not dependent on normal serum or plasma water percentage and those that do not require a quantitative blood serum or plasma dilution do not generate these artifacts. Devices that use direct ion selective electrode (ISE) methodology (for example, most "point of care" instruments and electrolyte measurements carried out by blood gas analyzers use direct ISE) are not affected by this artifact.
●Severe hypernatremia (serum sodium concentration above 170 mEq/L). Electrolyte analyzers are not calibrated for these extreme ranges. Therefore, although the sodium measurement is in the supernormal range, it may be lower than the "true" value.
●Pseudohyperchloremia can be generated by the following conditions:
•Marked hyperlipidemia can falsely elevate the chloride measurements when certain colorimetric chloride assays are used. This is generated when optically dense lipids cause light scattering and thereby lead to marked overestimation of the plasma chloride concentration, occasionally to levels above 200 mEq/L [51].
•Salicylate intoxication can falsely elevate chloride (pseudohyperchloremia) when high salicylate levels alter the permeability of certain ion-selective electrodes used for chloride measurement [5,40,41,45]. When salicylate intoxication has generated a high AG metabolic acidosis, the pseudohyperchloremia will result in a calculated serum AG that is "falsely" reduced to normal or into the negative range [42,43,45]. Whenever an otherwise unexplained negative anion gap is reported, a salicylate level should be ordered [45].
•When bromide ingestion generates detectable serum bromide levels, marked pseudohyperchloremia can result. Bromide ions are measured as chloride by most clinical chloride analyzers; however, bromide reacts more strongly than chloride. Thus, each bromide ion is reported as three or more chloride ions. Consequently, a serum bromide of 5 mEq/L may be measured as an additional 15 or 20 mEq/L of "chloride" [5,52-55].
This phenomenon was encountered relatively commonly in the past when bromide salts were used as sedatives. They included over-the-counter medications, such as Bromo-Seltzer and Miles Nervine. Inorganic bromide has now been removed from most medications but is still present in pyridostigmine bromide, which is used for the treatment of myasthenia gravis, dextromethorphan hydrobromide, and some herbal medications [3,4,52-55]. A study of patients taking pyridostigmine bromide, for example, found that their AG was 8 mEq/L lower than in healthy individuals [52]. Bromide salts, such as potassium bromide, are also occasionally used to treat refractory seizure disorders [56].
SUMMARY
●Increased serum anion gap (AG) – The serum AG can be increased in conditions unrelated to an organic metabolic acidosis, a finding that may be of diagnostic importance (calculator 1). The clinical conditions that cause this include hyperalbuminemia, hyperphosphatemia, anionic paraproteins (usually IgA monoclonal immunoglobulins), metabolic alkalosis, a sporadic laboratory error, or a reproducible artifactual increase in the sodium concentration or decrease in the chloride and/or bicarbonate concentrations. (See 'High serum anion gap' above.)
●Low serum AG – A low serum AG is usually caused by hypoalbuminemia but may also be produced by a severe normal AG (hyperchloremic) metabolic acidosis or an increase in the concentration of unmeasured cations (eg, hyperkalemia, hypermagnesemia, lithium intoxication, or IgG monoclonal proteins in someone with multiple myeloma) (table 1). (See 'Low serum anion gap' above.)
●Negative serum AG – A negative serum AG is usually due to a sporadic laboratory error. A reproducible negative serum AG may be generated by marked hyperlipidemia, salicylate intoxication, or bromide ingestion (table 1). (See 'Negative serum anion gap' above.)
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