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anion gap

Na+ - (Cl- + HCO3-) Reference interval: - 6-12 meq/L (reference interval) Notes: - unmeasured cations Ca+2, Mg+2 & globulins average 14 meq/L - unmeasured anions, H2PO4-, SO4-2, albumin & organic acids average 24 meq/L Principle: 1) when the primary disturbance is a metabolic acidosis, a high anion gap indicates the presence of unmeasured anions 2) when the primary disturbance is other than metabolic acidosis, an increased anion gap reveals a hidden metabolic acidosis 3) in anion gap acidosis, the expected ratio between the change in anion gap to the change in plasma [HCO3-] is 1/1 - 2/1; a) a ratio of < 1 suggests non-anion gap acidosis; b) a ratio of > 1 suggest concurrent metabolic alkalosis [3] Increases: 1) increase in unmeasured anions a) uremia with increased phosphate & sulfate b) ketotic states with increased organic acids - diabetic ketoacidosis - alcoholism (alcoholic ketoacidosis) - starvation - hyperemesis c) lactic acidosis d) toxin ingestion - methanol* - salicylate (aspirin overdose) - ethylene glycol* - paraldehyde - iron - isoniazid - cyanide - toluene e) increased albumin as in dehydration 2) decrease in unmeasured cations a) metabolic alkalosis leads to decrease in globulin charge b) hypocalcemia * suspect when serum bicarbonate < 10 meq/L & plasma osmolal gap > 10 mOsm/kg H20 [3] Decreases: 1) increase in unmeasured cations a) Li+ intoxication b) hypercalcemia c) hypermagnesemia c) multiple myeloma* d) polyclonal gammopathy e) polymixin B therapy (polycationic agent) f) increased NH4+ (early phase of chronic renal failure) 2) decrease in unmeasured anions a) hypoalbuminemia* b) bromide intoxication (interference with Cl- determination) c) hyponatremia with normal or increased extracellular fluid leads to loss of unmeasured cations * anion gap may < 4 meq/L [3]

Related

urinary anion gap

Specific

anion gap 3 anion gap 4

General

general chemistry test

References

  1. Clinical Diagnosis & Management by Laboratory Methods, 19th edition, J.B. Henry (ed), W.B. Saunders Co., Philadelphia, PA. 1996, pg 151 - Henry's Clinical Diagnosis & Management by Laboratory Methods, 21st edition, McPherson RA & Pincus MR (es), W.B. Saunders Co., Philadelphia, PA. 2007, pg 84
  2. Saunders Manual of Medical Practice, Rakel (ed), WB Saunders, Philadelphia, 1996, pg 1161
  3. Medical Knowledge Self Assessment Program (MKSAP) 14, 15, 16. 18, 19. American College of Physicians, Philadelphia 2006, 2009, 2012, 2018, 2021. - Medical Knowledge Self Assessment Program (MKSAP) 19 Board Basics. An Enhancement to MKSAP19. American College of Physicians, Philadelphia 2022
  4. Wilczynski C, Staros EB Medscape: Anion Gap http://emedicine.medscape.com/article/2087291-overview
  5. Wikipedia: Anion Gap http://en.wikipedia.org/wiki/Anion_gap
  6. Panel of 11 tests Laboratory Test Directory ARUP: 20144
  7. Panel of 9 tests Laboratory Test Directory ARUP: 20399
  8. Panel of 15 tests Laboratory Test Directory ARUP: 20408
  9. Panel of 5 tests Laboratory Test Directory ARUP: 20410
  10. Kraut JA, Nagami GT. The serum anion gap in the evaluation of acid-base disorders: what are its limitations and can its effectiveness be improved? Clin J Am Soc Nephrol. 2013 Nov;8(11):2018-24. Review. PMID: 23833313 Free PMC Article

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