The purpose of this study was to assess whether the concentration of serum
chloride and other variables, namely serum
sodium,
potassium and
bicarbonate, can be used to predict metabolic
acid-base status in infants with
hypertrophic pyloric stenosis (HPS) and to assess whether such a prediction is influenced by the state of hydration. One hundred and sixty-three infants with HPS who had at least one set of serum
electrolyte and capillary blood gas estimations performed after admission were studied retrospectively. A further 25 infants who had their
electrolyte and blood
gases measured after at least 12 h of
rehydration and correction of the serum
sodium and
chloride abnormalities were studied prospectively. Stepwise multiple regression analysis, using standard
bicarbonate as the dependent variable, revealed serum
chloride concentration to be the most powerful independent predictor of standard
bicarbonate level (r = -0.69, P less than 0.0001). Other variables did not improve the correlation significantly. In the retrospective study of untreated patients, the calculation of a 90 per cent prediction interval for the model indicated that if the serum
chloride level is less than 96 mmol/l, one could be 95 per cent confident (one tail) that the patient was alkalaemic. To be 95 per cent confident that the patient was not alkalaemic, the serum
chloride level would have to be greater than 121 mmol/l. The prospective study found that following
rehydration a serum
chloride level greater than or equal to 106 mmol/l more accurately predicted absence of alkalaemia. We conclude that predictability of
acid-base status from measurement of serum
chloride depends on the state of hydration of the patient.