A 23-year-old man with a history of type 1 diabetes mellitus is brought to the emergency department due to confusion and weakness. His symptoms began 2 days ago after he started having mild diarrhea. He has missed several doses of insulin because his appetite has been poor. On examination, his breath has a fruity odor. This patient is most likely to demonstrate which of the following urine chemistry patterns?

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11ec160b_3cf0_3857_9249_199c192a5b88_MD0001_00 This patient has diabetic ketoacidosis, which (like other forms of metabolic acidosis) is characterized by decreased serum pH and bicarbonate (HCO₃^-) with a compensatory decrease in pCO₂. The kidneys try to correct metabolic acidosis via 3 major mechanisms: Increased HCO₃^- reabsorption: Carbonic anhydrase in the proximal tubular lumen facilitates reabsorption of filtered HCO₃^-. Each reabsorbed HCO₃^- is equivalent to secretion of a H⁺. In states of metabolic acidosis, HCO₃^- is completely reabsorbed from the tubular fluid. Increased H⁺ secretion: Acidosis increases H⁺ secretion throughout the nephron. However, pH changes rapidly with relatively small changes in H⁺ concentration, limiting the amount of acid that can be secreted as free H⁺ in the urine. Increased acid buffer excretion: In order to facilitate excretion of much larger amounts of acid, the kidney utilizes acid buffers to trap H⁺ without markedly lowering urinary pH. The 2 most important acid buffers in urine are hydrogen phosphate (HPO₄^2-) and ammonia (NH₃), which combine with secreted H+ to form H₂PO₄^- and NH₄⁺. In chronic acidosis, proximal tubular cells greatly increase production of NH₃ to increase acid excretion. Educational objective: The kidneys compensate for metabolic acidosis by completely reabsorbing filtered bicarbonate (HCO₃^-) and excreting excess H⁺ in the urine. Most of the excreted H⁺ is buffered by phosphate (H₂PO₄^-) and ammonium (NH₄⁺), which allows for large amounts of acid to be excreted without precipitously dropping the pH.

A 23-Year-Old Man with a History of Type 1 Diabetes