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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 |
osmolal gap |
Suggests |
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high |
normal |
salicylate or paraldehyde |
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high |
high |
methanol or ethylene glycol |
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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