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Overview :
The electrolytes in the serum must be in equilibrium. The
major positively charged electrolytes (sodium, potassium) and negatively
charged electrolytes (chloride and bicarbonate) are commonly measured in
screening chemical tests. "Unmeasured" anions or cations make up the
difference from electroneutrality.
Method 1
anion gap =
= (sodium in mEq/L) - ((chloride in mEq/L) + (HCO3 in
mEq/L))
normal: 8-16 mEq/L
Method 2
anion gap =
= ((sodium in mEq/L) + (potassium in mEq/L)) - ((chloride in
mEq/L) + (HCO3 in mEq/L))
normal: 10-20 mEq/L
Interpretation
Increases in anion gap are seen with "unmeasured"
anions. In order of frequency these are:
(1) diabetic ketoacidosis
(2) uremic acidosis
(3) drug ingestion (salicylates, methanol, ethylene glycol,
ethanol, penicillins)
(4) lactic acidosis
(5) decreased
unmeasured cations (hypokalemia, hypocalcemia, hypomagnesemia)
(6) other
(hyperalbuminemia, elevated phosphorus, administration of sulfates, laboratory
error)
A decreased
anion gap is less frequent
(1) decreased
unmeasured anions , especially hypoalbuminemia
(2) laboratory
error
(3) increased
immunoglobulins (myeloma, gammopathies)
(4) increased
unmeasured cations (hyperkalemia, hypercalcemia, hypermagnesemia, lithium
therapy)
The anion gap
can be interpreted in conjunction with the osmolal gap.
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anion gap
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osmolal gap
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Suggests
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high
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normal
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salicylate or paraldehyde
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high
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high
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methanol or ethylene glycol
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The serum anion gap is affected by the chemical analyzer
used, so that the normal range needs to be determined for each institution.
• Some chemical
analyzers have a lower normal range that overlaps with that seen with other
analyzers.
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