Editor—Matos et al 1 have used indirect measurements in their recent paper to conclude that the urinary excretion of solutes cannot be evaluated by factoring them against urinary creatinine in the first week of life. They say they “would have preferred to compare the reported spot urine results with normal data obtained by complete, timed urine collection” because this would have given them “the ultimate proof of [their] assertion”, but seem unaware that such data already exist and that they contradict their conclusions.2-4

In the 1980s, we independently quantified creatinine excretion in 101 infants of 26–42 weeks gestation (and 640 to 4200 g birth weight) when 1.5–63 days old. In Newcastle, we used a constant inulin infusion technique5 to measure urine flow in 41 babies, some of whom required ventilatory support.3 In London, 24 hour urine collections were obtained from 60 stable babies using a continuously drained adhesive urine bag.4 Our measurements of daily urinary creatinine excretion were virtually identical. In Newcastle, we reported mean values of 104 μmol/kg in the first week of life and 95 μmol/kg in weeks 2 to 4, and in London we recorded an arithmetic mean of 96 μmol/kg throughout. In each case, the values were unaffected by gestational or postnatal age, or by the baby's weight centile.

Because urinary creatinine excretion per kilogram is constant from two days …