What acid-base disturbance is most common in CKD and why?

• A. Metabolic alkalosis due to bicarbonate retention.
• B. Respiratory alkalosis due to hyperventilation.
• C. Metabolic acidosis due to reduced hydrogen ion excretion and bicarbonate regeneration.
• D. Normal acid-base status due to compensatory mechanisms.

Answer: (C)

In CKD, metabolic acidosis is most common because the kidneys lose their ability to excrete acid and regenerate bicarbonate. As filtration falls, ammoniagenesis in the proximal tubule decreases, so less hydrogen ion is excreted as ammonium. At the same time, bicarbonate reabsorption and new bicarbonate generation are impaired, leading to a depletion of the bicarbonate base. The net result is accumulation of nonvolatile acids and a fall in bicarbonate, producing a hyperchloremic metabolic acidosis. The body can compensate by increasing ventilation to blow off carbon dioxide, but this respiratory response can’t fully normalize the pH, especially as kidney function continues to decline. Other acid-base patterns don’t fit the typical CKD picture because alkalosis would require bicarbonate retention, respiratory alkalosis involves low CO2 from hyperventilation driven by other factors, and a normal acid-base status would ignore the persistent acidosis seen with reduced renal function.