Dear editorImashuku et al1 describe a 77-year-old male patient with multiple myeloma who was admitted to the hospital after suffering a pelvic bone fracture due to a road traffic accident. Several days after admission the arterial blood gas showed a pH of 7.481; arterial carbon dioxide tension (PaCO2) of 28.2 mmHg; arterial oxygen tension (PaO2) of 84.0 mmHg; HCO3− of 20.8 mmol/L (normal; 23–31 mmol/L); and an anion gap of 8.9 mmol/L (normal;12 mmol/L). These data, as the authors concluded, were suggestive of metabolic acidosis. First, this is not true because a high pH and low PaCO2 confirm a respiratory alkalosis. Since the test was conducted days later we may expect a chronic respiratory alkalosis to be present, perhaps because of pain or a secondary pulmonary problem, as may be expected with a relatively low PaO2. In chronic respiratory alkalosis one would expect the HCO3− to decrease about 4 mmol/L with every 10 mmHg decrease of PaCO2.2 If the initial HCO3− had been about 25 mmol/L, the expected PaCO2 would be about 20.28 mmol/L, almost identical with the patient’s HCO3−.Second, because the authors erroneously considered this a case with a normal anion gap metabolic acidosis, they mistakenly considered the high urine anion gap of 48 mmol/L to be indicative of a distal renal tubular acidosis due to multiple myeloma. However, the urine anion gap may be useful in evaluating the renal ammonium excretion only in a normal anion gap metabolic acidosis. In respiratory alkalosis, the urine anion gap has no added value in this context.Third, a low anion gap is a feature of multiple myeloma, particularly of IgG myeloma, because the paraprotein is a cation.3Dear editorDr Berend’s critical comments on the blood gas analysis are greatly appreciated. Although respiratory alkalosis in multiple myeloma was reported in apparently dyspneic patients due to interstitial pneumonitis,1,2 we did not notice any respiratory distress in our patient. Thus, Dr Berend’s evaluation of our blood gas data was unexpected, but that helps in understanding the patient’s underlying pathophysiology. As pointed out, chronic pain might have affected respiratory conditions in our patient. We thank Dr Berend for his reminder on the complex acid-base balance in myelomapatients.