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Anion Gap

Total concentrations of cations and anions are normally equal, making serum electrically neutral. Measuring the gap between measured cation and anion levels provides information about the level of anions (including sulfate, phosphate, organic acids such as ketone bodies and lactic acid, and proteins) that are not routinely measured in laboratory tests. In metabolic acidosis, measuring the anion gap helps to identity the type of acidosis and possible causes. Further tests are usually needed to determine the specific cause of metabolic acidosis.

Purpose

• To distinguish types of metabolic acidosis.
• To monitor renal function and total parenteral nutrition

Patient preparation

• Explain to the patient that this test is used to determine the cause of acidosis.
• Tell him that the test requires a blood sample. Explain who will perform the venipuncture and when.
• Explain that he may experience slight discomfort from the needle puncture and the tourniquet but that collecting the sample usually takes less than 3 minutes.
• Inform him that he needn't restrict food or fluids before the test.
• Check the patient's history for use of medications that may influence sodium, chloride, and bicarbonate blood levels (such as diuretics, corticosteroids, and antihypertensives). If these medications must be continued, note this on the laboratory slip.

Procedure and posttest care

• Perform a venipuncture, and collect the sample in a 7-ml red-top or red­marble-top tube.
• If a hematoma develops at the venipuncture site, apply warm soaks.
• Instruct the patient to resume use of medications discontinued before the test.

Precautions

• Handle the sample gently to prevent hemolysis.

Reference values

• Normally, the anion gap is at least 12 mEq/L.

Abnormal findings

A normal anion gap does not rule out metabolic acidosis. It may occur in hyperchloremic acidoses, renal tubular acidosis, and severe bicarbonate-wasting conditions, such as biliary or pancreatic fistulas and poorly functioning ileal loops.

When acidosis results from loss of bicarbonate in the urine or other body fluids, the anion gap remains unchanged. This is known as normal anion gap acidosis.

An increased anion gap indicates an increase in one or more of the unmeasured anions (sulfate, phosphates, organic acids such as ketone bodies and lactic acid, and proteins). This may occur with acidoses that are characterized by excessive organic or inorganic acids, such as lactic acidosis or keto­acidosis.

When acidosis results from an accumulation of metabolic acids - as occurs in lactic acidosis, for example ­ the anion gap increases (above 12 mEq/L) with the increase in unmeasured anions. Metabolic acidosis caused by such an accumulation is known as high anion gap acidosis.

A decreased anion gap is rare but may occur with hypermagnesemia and paraproteinemic states, such as multiple myeloma and Waldenstrom's macroglobulinemia.

Interfering factors

• Hemolysis due to rough handling of the sample.
• Diuretics, lithium, chlorpropamide, and vasopressin (possible decrease due to decreased serum sodium levels)
• Corticosteroids and antihypertensives (possible increase due to increased serum sodium levels)
• Salicylates, paraldehyde, methicillin, dimercaprol, ammonium chloride, acetazolamide, ethylene glycol, and methyl alcohol (possible increase due to decreased serum bicarbonate level.)
• Corticotropin, cortisone, mercurial or chlorthiazide diuretics, and excessive ingestion of alkalis or licorice (possible decrease due to increased serum bicarbonate levels)
• Ammonium chloride, cholestyramine, boric acid, oxyphenbutazone, phenylbutazone, and excessive I.V. in, fusion of sodium chloride (possible dr crease due to increased serum chlorid levels)