Cystinuria Profile, Quantitative, 24 Hour, Urine
Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test
Cystinuria is an inborn error of metabolism resulting from poor absorption and reabsorption of the amino acid cystine in the intestine and in the kidney. This leads to an accumulation of poorly soluble cystine in the urine and results in the production of kidney stones (urolithiasis). Symptoms may include acute episodes of abdominal or lower back pain, presence of blood in the urine (hematuria), and recurrent episodes of kidney stones may result in frequent urinary tract infections, which may ultimately result in renal insufficiency. The combined incidence of cystinuria has been estimated to be 1 in 7,000.
Cystinuria can be classified into 3 subtypes based on the excretion of amino acids in the urine of heterozygotes (parents or children of affected individuals). Heterozygotes of type I excrete normal amounts of cystine, while those with types II and III present with slight to moderate excretion of cystine and other amino acids (lysine, arginine, and ornithine). All 3 subtypes are caused by mutations in only 2 genes, SLC3A1 on chromosome 2p and SLC7A9 on chromosome 19q. A new classification system has been proposed to distinguish the various forms of cystinuria: type A, due to mutations in the SLC3A1 gene; type B, due to mutations in the SLC7A9 gene; and type AB, due to 1 mutation in each SLC3A1 and SLC7A9.
Diagnosis of cystinuria
Homozygotes or compound heterozygotes with cystinuria excrete large amounts of cystine in urine, but the amount varies markedly. Urinary excretion of other dibasic amino acids (arginine, lysine, and ornithine) are also typically elevated. Plasma concentrations are generally normal or slightly decreased.
Individuals who are homozygous and heterozygous for non-type I cystinuria can be distinguished by the pattern of urinary amino acids excretion: the former secretes large amounts of cystine and all 3 dibasic amino acids, whereas the latter secretes more lysine and cystine than arginine and ornithine.
Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances
No significant cautionary statements.
Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.
3-15 years: 11-53 mcmol/24 hours
> or =16 years: 28-115 mcmol/24 hours
3-15 years: 19-140 mcmol/24 hours
> or =16 years: 32-290 mcmol/24 hours
3-15 years: 3-16 mcmol/24 hours
> or =16 years: 5-70 mcmol/24 hours
3-15 years: 10-25 mcmol/24 hours
> or =16 years: 13-64 mcmol/24 hours
Result in mcmol/24 hours x 0.24=result in mg/24 hours
Result in mg/24 hours x 4.17=result in mcmol/24 hours
Clinical References Provides recommendations for further in-depth reading of a clinical nature
1. Knoll T, Zollner A, Wendt-Nordahl G, et al: Cystinuria in childhood and adolescence: recommendations for diagnosis, treatment, and follow-up. Pediatr Nephrol 2005 January;20(1):19-24
2. Cystinuria. In The Metabolic and Molecular Bases of Inherited Disease. Eighth edition. Edited by CR Scriver, AL Beaudet, WS Sly, et al. New York, McGraw-Hill Book Company, 2001, pp 4909-4932