How does urine pH influence ion trapping of weak acids and bases, and what is the clinical implication?

• A. Weak acids are more ionized in basic urine and excreted more quickly, weak bases ionize in acidic urine and are trapped; urine pH can be manipulated to enhance clearance of certain drugs
• B. Weak acids trapped in basic urine; weak bases excreted quickly in acidic urine
• C. pH has no effect on ion trapping
• D. Ion trapping occurs only for proteins

Answer: (A)

Urine pH shifts the balance between the ionized and non-ionized forms of drugs. The kidney tubule reabsorbs mainly the non-ionized form, so making urine more ionized traps the drug in the urine and promotes excretion.

For a weak acid, increasing urine pH (more alkaline) makes it ionized, so it is less likely to be reabsorbed and is excreted more quickly. For a weak base, decreasing urine pH (more acidic) ionizes it, trapping it in the urine and increasing its excretion.

Clinically, this means urine pH can be manipulated to accelerate clearance of certain drugs or toxins. Alkalinizing urine helps eliminate weak acids (such as salicylates), while acidifying urine can help eliminate certain weak bases. This approach is used selectively and with medical supervision due to potential systemic effects.