media (do not use culture media if planning for microscopic studies), normal saline without preservative, or normal saline–soaked sterile gauze in sterile tube, freeze at –70°C Fibroblast culture provides unlimited specimen Organ biopsy Brain b Heart muscle b Liver a cm3 , 10–20 mg, ≤0.5 cm thick Kidney b Histochemical light and/or electron Biopsy potentially affected organs, microscopy collect within 1–2 hrs after death Enzyme activity Biochemical metabolites Mitochondrial studies Spleen b Skeletal muscle 20–50 mg, ≤0.5-cm thick Bile Bile mL Needle or open incisional biopsy, sterile technique, wrap in aluminum foil, dry ice, freeze at −70°C, screw-top airtight vial Some assays may need to be performed on fresh specimens Bile acids Acylcarnitines a If family declines autopsy but gives permission for specimen collection, or if unable to obtain autopsy within hours of death, collect blood, urine, and CSF; perform punch or open incisional biopsy of skin and needle biopsy of liver and skeletal muscle; take photographs of dysmorphic features; and obtain radiologic studies to evaluate for neurologic, cardiac, or skeletal abnormalities Obtain parental permission Tests that are not accurate using postmortem specimens are those for serum amino acids, lactate, pyruvate, and total and free carnitine assessment Consider developing postmortem specimen collection kit for ED that contains necessary equipment, specimen containers, and institution-specific instructions b Obtain an autopsy if autopsy permission granted EDTA, ethylenediaminetetraacetic acid; PCR, polymerase chain reaction; ED, emergency department Hyperammonemia is the hallmark of urea cycle defects but also occurs in organic acidemias and fatty acid oxidation defects as a consequence of secondary inhibition of the urea cycle Ammonia levels are typically highest in urea cycle defects and may exceed 1,000 μg/dL Ammonia levels in organic acidemias are usually less than 500 μg/dL during decompensation but may exceed 1,000 μg/dL Hyperammonemia in fatty acid oxidation defects, if present, is usually less than 250 μg/dL Transient hyperammonemia of the newborn should be considered in the differential diagnosis, particularly if hyperammonemia is present on the first day of life Hyperammonemia directly stimulates the respiratory center, resulting in tachypnea Ammonia level higher than 250 μg/dL with respiratory alkalosis in the absence of metabolic acidosis is highly suggestive of a urea cycle defect Proper collection and handling of blood for ammonia determination is critical to prevent falsely elevated values Abnormal levels should be confirmed immediately using proper technique for drawing and handling Patients with urea cycle defects may have compensatory metabolic acidosis Patients with organic acidemias and fatty acid oxidation defects and hyperammonemia have primary metabolic acidosis usually without respiratory alkalosis Patients with hyperammonemia due to organic acidemias usually have marked ketosis and normal glucose level, whereas those with fatty acid oxidation defects usually have hypoketotic hypoglycemia Even during minor illnesses, protein catabolism may result in hyperammonemia In patients with hyperammonemia, liver function should be evaluated Mild elevation of transaminases may be seen in metabolic disorders in each category Plasma should be sent for amino