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?
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?
💡 Explanation
**Core Concept:** Anion gap is a measure of the difference between the total anions (negatively charged ions) and cations (positively charged ions) in blood. It is calculated using serum electrolytes, including sodium, potassium, chloride, and bicarbonate. An increased anion gap indicates the presence of unmeasured anions, such as lactic acidosis or acetoacetate acidemia, while a decreased anion gap suggests normal or low anion levels, such as in hyponatremia or hypokalemia.
**Why the Correct Answer is Right:** In this scenario, we are dealing with a patient who presents with left-sided flank pain and microscopic hematuria, but no other concerning findings. The anion gap is calculated as follows:
Anion gap = (Na + + K + + (Cl - + HCO3-) - (Na+ + K+)
A. This option is too high (38 mEq/L), indicating a low anion gap, which is not consistent with the patient's clinical presentation.
B. The anion gap (37 mEq/L) is within the normal range (10-20 mEq/L), suggesting that this option is not correct.
C. The patient's anion gap (37 mEq/L) is too high, suggesting a low anion gap, which does not match the patient's clinical picture.
D. The anion gap (37 mEq/L) is within the normal range, indicating that this option is not suitable for the patient's condition.
**Clinical Pearl:** An elevated anion gap is often seen in conditions like diabetic ketoacidosis, lactic acidosis, or certain congenital disorders like galactosemia. A normal anion gap indicates conditions like hypokalemia, hyponatremia, or hypochloremia. A decreased anion gap is seen in conditions like hypokalemia, hyponatremia, or hypochloremia.
**Answer:** The patient's anion gap is elevated (37 mEq/L). This suggests the presence of lactic acidosis, which is a form of acid-base imbalance. Lactic acidosis can occur due to various causes like sepsis, trauma, or major surgery. The patient's clinical presentation (left-sided flank pain and left-sided renal colic) and laboratory findings (microhematuria) support the clinical suspicion of renal colic. This is a common urological problem involving the kidney stones. A high anion gap indicates the presence of unmeasured anions like lactate, which is
✓ Correct Answer: C. 13.4 mEq/L
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