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, it must be emphasized that this is merely a clue, is not always present, may occur in early fasting, and is, therefore, quite unreliable. Nonetheless, when present, it should trigger a search for hyperammonemia.
Liver function is normal unless there has been hypoxia in association with seizures.
The same is true of renal function. Electroencephalogram will show non-specific, diffuse encephalopathy. Lumbar puncture should be approached with great caution because of the potential for cerebral edema. The sole laboratory criterion for early diagnosis is a blood ammonia level, which should be obtained simultaneously with the usual laboratory work-up as a part of the investigation of a critically ill neonate or infant. Adults with unexplained lethargy or coma should also have blood ammonia determined.