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Lab Test: Anion Gap

    Lab Test
    • Anion Gap
    • Measurement of the difference between anions and cations in plasma or serum used in the evaluation of acid-abase disorders.
    Reference Range
    • Adults and Children:
      • If potassium (K+) is used in the calculation:  16 ± 4 mEq/L  or range of 10-20 mEq/L (10-20 mmol/L)
      • If potassium (K+) is not used in the calculation:  12 ± 4 mEq/L or range 7-16 mEq/L (7-16 mmol/L)
    Indications & Uses
    • Initial evaluation and monitoring of diabetic ketoacidosis (DKA):
      • Anion gap is > 10 mEQ/L in mild DKA and > 12 mEq/L in moderate to severe cases.
      • After initial evaluation, calculate the anion gap every 2 to 4 hours to monitor resolution of acidosis.
    • Initial evaluation of suspected hyperglycemic hyperosmolar state (HHS):
      • About 50% of patients have an increased anion gap metabolic acidosis due to concomitant ketoacidosis and/or an increases in serum lactate levels.
    • Suspected alcoholic ketoacidosis
    • Suspected hypermagnesemia
    • Suspected lactic acidosis
    • Suspected metabolic acidosis
    • Other conditions other than metabolic acidosis that could cause an elevated anion gap are:
      • Dehydration, treatment with sodium salts of strong acids, treatment with sodium salts of antibiotics (e.g., penicillin, carbenicillin), alkalosis, decreased unmeasured cation (e.g., hypokalemia, hypocalcemia, hypomagnesemia), hyperalbuminemia, increased inorganic anion (e.g., phosphate, sulfate), laboratory error (e.g., falsely increased serum sodium falsely decreased serum chloride or bicarbonate).
    • The anion gap may be lowered by one of three mechanisms:
      • Increased unmeasured cation (e.g., hyperkalemia, hypercalcemia, hypermagnesemia, multiple myeloma, lithium intoxication, polymyxin B)
      • By decreased unmeasured anion (e.g., hypoalbuminemia or bromide (Br-) intoxication) By laboratory error (e.g., falsely decreased serum sodium, falsely increased serum chloride or bicarbonate hyperviscosity, hyperlipidemia, dilutional studies).  Calculation of the anion gap should be adjusted for albumin.  The anion gap falls by approximately 2.5 mgEq/L for every 1 g/dL reduction in serum albumin concentrations.
    Clinical Application
    • The anion gap is the measurement in the difference between the cations and the anions in the extra-cellular space.  Although anion gap can be measured with or without potassium (K+), the historical standard is to measure without K+ since its variation is clinically insignificant.
    • This calculation is most often helpful in identifying the cause of metabolic acidosis.
    Related Tests
    • Electrolytes, potassium, chloride, bicarbonate measurement are necessary to calculate the AG.
    • Arterial blood gases (ABG)
    Drug-Lab Interactions
    • The anion gap falls by approximately 2.5 mEq/L for every 1 g/dL reduction in serum albumin concentration.
    • Hyperlipidemia may cause under measurement of Na+ (sodium) and falsely decrease AG.
    • Normal values of AG vary according to different normal values for electrolytes, depending on laboratory methods of measurement.
    • Drugs that increase the AG include:  carbenicillin, carbonic anhydrase inhibitors (e.g., acetazolamide), diuretics, ethanol, methanol, penicillin, and salicylate.
    • Drugs that decrease AG may include acetazolamide, lithium, polymyxin B, spironolactone, and sulindac.
    Test Tube Needed
    • Red or green top tube
    • Collect a venous blood sample.  If the patient is receiving an intravenous infusion, obtain the blood from the opposite arm.
    • The AG is then calculated.
    Storage and Handling
    • Increased results will occur when the serum sample is exposed to air for excessive periods of time.
    What To Tell Patient Before & After
    • Explain the procedure to the patient and tell them that no food or fluid is restricted.
    1. LaGow B et al., eds. PDR Lab Advisor. A Comprehensive Point-of-Care Guide for Over 600 Lab Tests.  First ed. Montvale, NJ: Thomson PDR; 2007.
    2. Pagana K, Pagana TJ eds. Mosby's Manual of Diagnostic and Laboratory Tests. 5th Ed.  St. Louis, Missouri. 2014.
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MESH Terms & Keywords

  • Anion Gap