blood ammonia. ammonia is a cerebrointoxicant and a high blood level causes a degenerative brain lesion. High blood levels of ammonia can occur in a number of diseases of the liver, in portacaval shunts, urea poisoning and liver dysfunction. ammonia clearance. see ammonia tolerance test (below).mmonia is a cerebrointoxicant and a high blood level causes a degenerative brain lesion. High blood levels of ammonia can occur in a number of diseases of the liver, in portacaval shunts, urea poisoning and liver dysfunction.