Bilirubin Measurement Transcutaneous measurements of bilirubin are correlated with serum bilirubin; however, they are inaccurate at higher levels (greater than 12 to 15 mg/dL), and thus are best used as a screen A TSB should always be obtained when therapeutic intervention is being considered Nearly all published data regarding the correlation of TSB levels to kernicterus or developmental outcome are based on capillary blood Data on the relationship between capillary and venous sampling are conflicting Capillary sampling is endorsed by the AAP; a confirmatory venous sample is not required In neonates, it may be important to determine the rate of rise of TSB with serial measurements It is imperative to note that many clinical laboratories require the total and fractionated bilirubin to be ordered separately, as the total bilirubin reported on the hepatic function or comprehensive metabolic panels is unreliable in infants under month of age The ED clinician should be familiar with the accuracy of his or her laboratory assay in order to minimize error in the evaluation and management of neonates with suspected hyperbilirubinemia Other Laboratory Studies If the TSB level is below 12 mg/dL, rises slowly, and resolves before days of age, one can diagnose physiologic hyperbilirubinemia without further laboratory studies When these conditions are not met, further testing is required to determine the etiology of elevated serum bilirubin A complete blood cell count should be obtained to evaluate for anemia A peripheral blood smear should be examined microscopically for clues as to the etiology of the anemia: characteristic abnormal morphology, such as sickle cells, spherocytes, or elliptocytes, may be identified; helmet and fragmented cells are diagnostic of a microangiopathic hemolytic anemia; malarial ring forms may be apparent The reticulocyte count may be elevated in the setting of hemolysis Patients with anemia or hemolysis should also have a Coombs test performed to look for evidence of autoimmune hemolysis In patients with a TSB level above threshold for exchange transfusion, a serum albumin should be obtained, and ratio of bilirubin to albumin should be calculated End-tidal carbon monoxide concentration (ETCOc) provides a noninvasive assessment of bilirubin production, and may be utilized to aid in confirmation of active hemolysis The child with fever, hypothermia, or ill appearance should be evaluated for serious bacterial infection, including blood, urine, and cerebrospinal fluid cultures as indicated Serum electrolytes should be obtained in patients with clinical signs of dehydration, and those with a history of emesis or excessive stool output