Cystine (Urine)

General Information

Cystinuria is a classic example of an amino aciduria due to a true defect in renal tubular function, in that the amino aciduria occurs at normal or even low plasma concentrations of the acid involved.

In most cases of cystinuria there is renal loss not only of cystine but also of the dibasic amino acids ornithine, arginine and lysine. There is also an associated failure of intestinal absorption of the same amino acids.

The only known clinical manifestation of cystinuria is recurrent urinary tract stone formation. Cystine stones form readily in acidic urine. They are yellow-brown in colour and are radio-opaque because of their sulphur content. Patients with cystinuria also have a higher incidence of cacium oxalate stones than the general population. All stone formers may therefore benefit from screening for cystinuria by either qualitative screening or formal amino acid measurement of the urine.