The rising costs of health care and the movement for health care reform have focused attention on methods of cost containment. Of routine laboratory and radiologic procedures, complete blood cell count (CBC) and determination of serum electrolyte values rank as high as 2nd and 9th in overall cost. We retrospectively studied use of the clinical laboratory to aid diagnosis of an acute infectious event in a pediatric emergency department population.For 5 months, we reviewed medical records of pediatric patients younger than 15 years brought to the emergency department because of a febrile episode. Of 155 cases reviewed, electrolyte concentrations were determined in 108 patients and CBC in 155. In all patients, either culture or rapid test for streptococcal organisms was performed. In addition, 838 pediatric patients with similar symptoms but who did not undergo laboratory testing were monitored for 100 days. Measures of effectiveness including sensitivity, specificity, positive and negative predictive values, and likelihood ratio were used to correlate specific laboratory findings with antibiotic therapy, serious bacterial disease, and culture positivity.Electrolyte abnormalities were found largely to be dismissed clinically, with the major clinical response consisting of parental education about hydration. The CBC profile was evaluated, with white blood cell count (WBC) indicator limits of >10,000, >10,000 but <15,000, and >15,000/mm 3 , and differentiated into absoluteThe rising costs of health care and calls for health care reform have focused attention on methods of cost containment. 1-8 Standards of care have been promulgated for specific clinical situations, in the hope of eliminating unnecessary laboratory testing and decreasing the use of tests with low yield, without compromise of patient care.9-11 Use and overuse of