A 45-year-old man presents to his doctor with complaints of left-sided flank pain radiating into his groin. The pain began suddenly and has increased in intensity. He had a similar problem several years ago, but he did not seek medical attention, and his symptoms resolved spontaneously. He denies fever, chills, nausea, or vomiting. He has had no diarrhea or difficulty urinating. Physical examination is remarkable for tenderness along the left flank and into the groin, but is otherwise normal. There are no inguinal hernias detected on examination. A urinalysis reveals microscopic hematuria without casts. Serum chemistry reveals: Sodium: 143 mEq/L Potassium: 2.4 mEq/L Bicarbonate: 17 mEq/L Chloride: 115 mEq/L Blood urea nitrogen: 12 mg/dL Creatinine: 1.0 mg/dL Glucose: 85 mg/dL Which of the following represents this patient's anion gap?
The concept of the anion gap is based on the idea that all the charges in a solution must add to zero, since the solution is electrically neutral. In practice, people just take the two principal cations, Na+ and K+, and the two principal anions, Cl- and HCO3-, and subtract the sum of the anions from the sum of the cations. When the correct formula is used, the anion gap is (+)-(+) = 143 + 2.4 - (17 + 115) = 13.4 mEq/L, which is within the normal range of 10-20 mEq/L when this formula is used for the calculation. (Some authors "cheat" even fuher and ignore the K+ as well, which works on the same principal, but produces a slightly different reference range of values.) The anion gap is due to the excess amount of unmeasured anions in the serum, when compared to the unmeasured cations. Ref: Stern S.C., Cifu A.S., Altkorn D. (2010). Chapter 4. I Have a Patient with an Acid-Base Abnormality. How Do I Determine the Cause?. In S.C. Stern, A.S. Cifu, D. Altkorn (Eds), Symptom to Diagnosis: An Evidence-Based Guide, 2e.
3.4 mEq/L
8.4 mEq/L
13.4 mEq/L
21.4 mEq/L