The most important step in diagnosing urea cycle disorders is clinical suspicion of hyperammonemia. A blood ammonia level is the first laboratory test in evaluating a patient with a suspected urea cycle defect. Particular care should be taken in drawing a blood ammonia since there is significant variability depending on proper technique and handling. The clinician should remember that treatment should not be delayed in efforts to reach a final diagnosis, and that later stages of treatment should be tailored to the specific disorder.
In addition to plasma ammonia, laboratory data useful in the diagnosis of UCDs include pH, CO2, the anion gap, blood lactate, plasma acylcarnitine profile acylcarnitines, plasma and urine amino acids, and urine organic acid analyses including the specific determination of orotic acid. Patients with true urea cycle defects will typically have normal glucose and electrolyte levels. The pH and CO2 can vary with the degree of cerebral edema and hyper- or hypo-ventilation.
In neonates it should be remembered that the basal ammonia level is elevated over that of adults, which typically is less than 35 µmol/L (less than 110 µmol/L in neonates). An elevated plasma ammonia level of 150 µmol/L (>260 µg/dl) or higher in neonates and >100 µmol/l (175 µg/dl) in older children and adults, associated with a normal anion gap and a normal blood glucose level, is a strong indication for the presence of a urea cycle defect. Quantitative plasma amino acid analysis can be used to evaluate these patients and arrive at a tentative diagnosis. Elevations or depressions of the intermediate amino-containing molecules arginine, citrulline, and argininosuccinate Click to view Diagnostic Flow Chart will give clues to the point of defect in the cycle.
The amino acid profile in sick newborns can be quite different from those in children and adults, which should be taken into account. The levels of the nitrogen buffering amino acid glutamine will also be quite high and can serve as confirmation of the hyperammonemia. If a defect in NAGS, CPSI, or OTC is suspected, the presence of the organic acid orotic acid in the urine can help distinguish the diagnosis. Orotic acid is produced when there is an overabundance of carbamyl phosphate which spills into the pyrimidine biosynthetic system. The determination of urine organic acids and plasma acylcarnitines will also herald the presence of an organic aciduria. Other genetic defects that affect ammonia detoxification are lysinuric protein intolerance, the hyperinsulinism-hyperammonemia syndrome, hypoprolinemia (paradoxical fasting hyperammonemia) and pyruvate carboxylase deficiency. All of them are very rare and important hints would be obtained through the investigations outlined above.
Enzymatic and genetic diagnosis is available for all of these disorders. For CPSI, OTC, and NAGS, enzymatic diagnosis is made on a liver biopsy specimen freshly frozen in liquid nitrogen. Enzymatic testing for ASS, ASL can be done on fibroblast samples and arginase can be tested on red blood cells. Clinically approved DNA sequence analysis is only available for OTC at the time of this printing, but its availability for the other disorders is anticipated soon, as it is available outside the US. A frequently updated web resource for testing information can be found at the NIH sponsored site: http://www.geneclinics.org
