A 35-year-old woman comes to the clinic reporting progressive fatigue, diffuse muscle weakness, and excessive thirst for the past three weeks. Her medical history is notable for Sjögren syndrome and two episodes of calcium phosphate nephrolithiasis over the past five years. She currently takes no medications. Her temperature is 37.0°C (98.6°F), blood pressure is 110/70 mmHg, pulse is 88/min, and respiratory rate is 16/min. Physical examination reveals dry mucous membranes but is otherwise unremarkable. Laboratory studies are obtained as shown below:
Serum Laboratory Results:
| Test | Result | Reference Range |
|---|---|---|
| Sodium (Na⁺) | 139 mEq/L | 136-145 mEq/L |
| Potassium (K⁺) | 2.7 mEq/L | 3.5-5.0 mEq/L |
| Chloride (Cl⁻) | 112 mEq/L | 98-106 mEq/L |
| Bicarbonate (HCO₃⁻) | 15 mEq/L | 22-28 mEq/L |
| Blood urea nitrogen (BUN) | 14 mg/dL | 7-20 mg/dL |
| Creatinine | 0.9 mg/dL | 0.6-1.2 mg/dL |
| Glucose | 88 mg/dL | 70-100 mg/dL |
Urine Laboratory Results:
| Test | Result |
|---|---|
| pH | 6.5 |
| Sodium (Na⁺) | 40 mEq/L |
| Potassium (K⁺) | 25 mEq/L |
| Chloride (Cl⁻) | 50 mEq/L |
| Urea Nitrogen (UUN) | 420 mg/dL |
| Glucose | 0 mg/dL |
| Measured Osmolality | 310 mOsm/kg H₂O |
Based on the laboratory data provided, what is the patient's calculated urine osmolar gap? (Enter the numerical value only)
Answer: 30 mOsm/kg
Answer
30
The patient has a normal anion gap metabolic acidosis (NAGMA) secondary to distal (type 1) renal tubular acidosis, which is strongly associated with Sjögren syndrome and calcium phosphate nephrolithiasis. In the workup of NAGMA, it is critical to determine if the kidneys are appropriately secreting acid in the form of ammonium (). The urine osmolar gap (UOG) is the most accurate indirect measure of urinary ammonium. The gap is the difference between the measured urine osmolality and the calculated urine osmolality.
The formula for calculated urine osmolality is:
Plugging in the patient's values:
The UOG is the measured osmolality minus the calculated osmolality:
A UOG of less than 40-50 mOsm/kg indicates low urinary ammonium excretion, confirming a renal etiology for the acidosis (RTA), whereas a UOG > 100 mOsm/kg would indicate adequate ammonium excretion (suggesting gastrointestinal bicarbonate loss).
The formula for calculated urine osmolality is:
Plugging in the patient's values:
The UOG is the measured osmolality minus the calculated osmolality:
A UOG of less than 40-50 mOsm/kg indicates low urinary ammonium excretion, confirming a renal etiology for the acidosis (RTA), whereas a UOG > 100 mOsm/kg would indicate adequate ammonium excretion (suggesting gastrointestinal bicarbonate loss).
Step-by-Step Solution
Key Concept
The urine osmolar gap is used to indirectly measure urinary ammonium excretion in the evaluation of a normal anion gap metabolic acidosis.