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Difficulty: MediumNephrology

A 38-year-old woman comes to the clinic reporting progressive muscle weakness and fatigue over the past several weeks. She has a past medical history of Sjögren syndrome, for which she uses artificial tears and takes hydroxychloroquine. She denies recent diarrhea, vomiting, or changes in her medications. Vital signs are within normal limits.

Laboratory studies show:
Sodium: 139 mEq/L139 \text{ mEq/L}
Potassium: 2.6 mEq/L2.6 \text{ mEq/L}
Chloride: 113 mEq/L113 \text{ mEq/L}
Bicarbonate: 14 mEq/L14 \text{ mEq/L}

Arterial blood gas on room air:
pH: 7.267.26
PaCO2: 29 mm Hg29 \text{ mm Hg}

Urinalysis reveals a pH of 6.86.8 with no glucose, protein, or blood.

Which of the following is the most likely primary mechanism for this patient's acid-base and electrolyte abnormalities?

  1. Defective hydrogen ion secretion in the distal tubuleAnswer
  2. B
    Decreased proximal tubular reabsorption of bicarbonate
  3. C
    Reduced collecting duct responsiveness to aldosterone
  4. D
    Excessive gastrointestinal loss of bicarbonate-rich fluids
  5. E
    Concomitant primary respiratory alkalosis

Answer

Defective hydrogen ion secretion in the distal tubule
The correct answer describes the pathophysiology of Type 1 (distal) renal tubular acidosis (RTA). This patient presents with a normal anion gap (12 mEq/L12 \text{ mEq/L}) metabolic acidosis. In the setting of a normal gap metabolic acidosis, the serum potassium and urine pH are critical for differentiating the etiology. The patient has hypokalemia, which rules out Type 4 RTA (characterized by hyperkalemia). To distinguish between gastrointestinal bicarbonate loss (diarrhea), Type 2 (proximal) RTA, and Type 1 (distal) RTA, the urine pH is evaluated. In a systemic acidemic state, a normal functioning distal nephron will maximize hydrogen ion secretion, lowering the urine pH below 5.5. The fact that this patient's urine pH is 6.8 indicates an inability of the distal tubule to secrete hydrogen ions. Furthermore, Type 1 RTA is strongly associated with autoimmune disorders, most notably Sjögren syndrome and rheumatoid arthritis.

Step-by-Step Solution

1
Calculate the serum anion gap to determine the type of metabolic acidosis.
Anion Gap = Na+−(Cl−+HCO3−)=139−(113+14)=12 mEq/LNa^+ - (Cl^- + HCO_3^-) = 139 - (113 + 14) = 12 \text{ mEq/L}. This is a normal anion gap (non-anion gap) metabolic acidosis.
Classifying the acidosis narrows the differential diagnosis between anion gap causes (e.g., ketoacidosis, lactic acidosis) and normal gap causes (e.g., renal tubular acidosis, diarrhea).
2
Calculate the expected respiratory compensation using Winter's formula.
PaCO2=1.5×[HCO3−]+8±2=1.5(14)+8=21+8=29±2 mm HgPaCO_2 = 1.5 \times [HCO_3^-] + 8 \pm 2 = 1.5(14) + 8 = 21 + 8 = 29 \pm 2 \text{ mm Hg}. The measured PaCO2PaCO_2 is 29 mm Hg29 \text{ mm Hg}, indicating appropriate compensation without a mixed disorder.
Verifying respiratory compensation rules out concurrent primary respiratory acidosis or alkalosis.
3
Evaluate the serum potassium and urine pH.
The patient has severe hypokalemia (2.6 mEq/L2.6 \text{ mEq/L}) and an inappropriately basic urine pH (6.86.8).
In normal gap metabolic acidosis, potassium distinguishes Type 4 RTA (high) from Types 1 and 2 RTA and diarrhea (low/normal). Urine pH distinguishes Type 1 RTA (> 5.5) from Type 2 RTA and diarrhea (< 5.5).
4
Synthesize the laboratory findings with the clinical presentation.
Normal anion gap metabolic acidosis + hypokalemia + urine pH > 5.5 + Sjögren syndrome indicates Type 1 (distal) renal tubular acidosis, caused by defective hydrogen ion secretion.
Autoimmune conditions like Sjögren syndrome are classic secondary causes of Type 1 RTA due to autoantibody-mediated damage to the distal intercalated cells.

Key Concept

Diagnosis of Type 1 (distal) Renal Tubular Acidosis
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Topics

Nephrology

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