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Background: Blood pH and bicarbonate estimations are basal acid-base laboratory tests that are performed in infants with infantile hypertrophic pyloric stenosis (IHPS). This study aimed to define the clinical value of pCO 2 and BE in infants suspected to have IHPS. Methods: We collected data from 80 "surgical" infants younger than 100 days with prolonged nonbilious vomiting who were suspected to have IHPS. In 65 infants, pyloric stenosis was confirmed, and 15 infants had nonsurgical conditions. Capillary blood was tested for standard acid-base parameters and lactate. The two groups were compared. Results: Eighty-eight percent of the IHPS infants had elevated standard bicarbonate levels (st bicarb) > 25 mmol/l, and 60% had BE > 3.5 mmol/l; 12% of the infants showed hypercapnia (pCO 2 ≥ 50 mmHg) associated with markedly increased standard bicarbonate and BE. Infants with nonsurgical vomiting were older at admission (p = 0.002), had a longer duration of vomiting (p < 0.001), were older (p = 0.002) and weighted more at admission (p = 0.004), had lower pCO 2 (p = 0.021), lower st bicarb (p < 0.001) and lower BE (p = 0.001). In addition, nonsurgical infants showed a trend to anemia (p = 0.002). Conclusions: In infants with IHPS/nonbilious vomiting, acid-base analysis (ABA) is equivocal or inconclusive. These findings may be misleading and could result in a false clinical decision. Nonsurgical vomiting is associated with a lower degree of alkalosis, normocapnia to slight hypercapnia and a base deficit. However, even infants with IHPS may present with a negative BE. In infants with IHPS and severe alkalosis, hypercapnia carries a risk for respiratory depression. Monitoring the infant's respi-

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