The enzyme deficiencies lead to disorders with nearly identical clinical presentations. Routine laboratory investigation is of no diagnostic help. The clue to the presence of CPS deficiency may be a subnormal blood-urea nitrogen level. In contrast to older textbooks, this is merely a clue, is not always present, may occur in early fasting, and is quite unreliable. Nonetheless, when present, subnormal blood-urea nitrogen level should trigger a search for hyperammonemia.
Liver function is normal unless hypoxia has occurred in association with seizures.
The same is true of renal function. Electroencephalogram will show nonspecific diffuse encephalopathy. Approach lumbar puncture with great caution because of the potential for cerebral edema. The sole laboratory criterion for early diagnosis is a blood ammonia level. Obtain blood level simultaneously with the usual laboratory workup as a part of the investigation of a critically ill neonate or infant.