Basis: metabolic alkalosis and hypochloremia associated with vomiting
Patient selection: infant with vomiting
Differential diagnosis: severe gastroesophageal reflux
Biochemical features identifying pyloric stenosis:
(1) serum bicarbonate >= 29 mmol/L
(2) serum chloride <= 98 mmol/L
Steps:
(1) Evaluate the infant for clinical evidence of pyloric stenosis (pyloric mass, with projectile vomiting, weight loss, acting hungry, etc.).
(2) Check biochemical values of serum and urine (which shows paradoxical aciduria).
(3) If the diagnosis is still in doubt or to confirm the biochemical values, perform ultrasonography.
Performance of the cutoffs:
• The sensitivity of bicarbonate is 99% and specificity is 36%.
• The sensitivity of bicarbonate is 99% and specificity is 50%.
