Ammonia, lactate and blood gases: a user's guide

Basic biochemical tests are frequently obtained in acutely unwell neonates and children, as well as in some elective situations. Correct interpretation is essential in identifying rare inherited primary metabolic disorders, but secondary causes of hyperammonaemia, elevated blood lactate or acid-base derangement are more common and require appropriate treatment of the underlying cause. Ammonia is the waste product of protein metabolism and is highly toxic. Ammonia should be measured in any sick neonate, and in children with unexplained encephalopathy. Further testing is needed to determine if it is secondary to other factors, or due to a primary metabolic disorder affecting urea cycle function. Specific treatment should be instigated urgently to avoid long term neurological sequelae. Lactate elevation indicates anaerobic respiration and is often secondary to hypoxia or poor tissue perfusion but can indicate a metabolic disorder affecting mitochondrial function or energy metabolism. Blood gas analysis to review acid-base status is a critical test in any sick neonate or child, and correct interpretation will indicate if there is a respiratory or metabolic basis. A metabolic acidosis with elevated anion gap may indicate a primary metabolic disorder. These tests can help identify patients who may have a primary metabolic disorder, and management should be discussed urgently with a specialist metabolic centre.