Anion Gap

Test Description

When electrolytes are evaluated, the substances being measured include two positive

ions, called cations, and two negative ions, called anions. The cations are

sodium (Na+) and potassium (K+), and the anions are chloride (Cl−) and bicarbonate

(HCO3−). When the total amount of cations and the total amount of anions are compared,

there are normally more cations than anions, leading to what is known as the

anion gap. This is because all of the possible anions are not measured. Those not

measured include organic acids, phosphates, and sulfates.

Measurement of the anion gap assists the primary care provider in determining

the potential causes of metabolic acidosis. Types of metabolic acidosis that have an

increased anion gap include those associated with renal failure, diabetic ketoacidosis,

and lactic acidosis.

The anion gap is calculated as follows:

(Na+ + K+) − (Cl− + HCO3−) = Anion Gap

Normal Values

12 ± 4 mEq/L (12 ± 4 mmol/L SI units) if potassium not included

16 ± 4 mEq/L (16 ± 4 mmol/L SI units) if potassium is included

Possible Meanings of Abnormal Values

Increased                                                                   Decreased

Alcoholic ketoacidosis                                               Bromide intoxication

Dehydration                                                               Hypercalcemia

Diabetic ketoacidosis                                                 Hyperdilution

Hypocalcemia                                                            Hypermagnesemia

Hypomagnesemia                                                      Hypoalbuminemia

Lactic acidosis                                                           Hyponatremia

Metabolic acidosis                                                     Hypophosphatemia

Renal failure                                                              Multiple myeloma

Salicylate toxicity

Uremia

Contributing Factors to Abnormal Values

• Hemolysis of the sample may alter test results.

• False decreases may occur owing to absorption of iodine from povidone-iodine

  packed wounds.

• Drugs that may increase the anion gap: acetazolamide, antihypertensives, carbenicillin,

corticosteroids, dextrose in water, dimercaprol, ethacrynic acid, furosemide, methyl

alcohol, nitrates, paraldehyde, penicillin, salicylates, sodium bicarbonate, thiazides.

• Drugs that may decrease the anion gap: antacids containing magnesium, boric acid,

chlorpropamide, cholestyramine, cortisone acetate, corticotropin, iodide, lithium

carbonate, phenylbutazone, polymyxin B, vasopressin.

Interventions/Implications

   Pretest

• Explain to the patient the purpose of the test and the need for a blood sample to be drawn.

• No fasting is required before the test.

  Procedure

• A 7-mL blood sample is drawn in a red-top collection tube for determination of the

  electrolytes.

• Gloves are worn throughout the procedure.

 Post test

• Apply pressure at venipuncture site. Apply dressing, periodically assessing for continued

  bleeding.

• Label the specimen and transport it to the laboratory.

• Report abnormal findings to the primary care provider.