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Introduction ⬇

Cystinuria is a condition characterized by increased amounts of the amino acid cystine in the urine. The presence of increased urinary cystine is caused not by a defect in the metabolism of cystine but rather by the inability of the renal tubules to reabsorb cystine filtered by the glomeruli. The tubules fail to reabsorb not only cystine but also lysine, ornithine, and arginine; this rules out the possibility of an error in metabolism, even though the condition is inherited.

These urine tests are useful for the differential diagnosis of cystinuria, an inherited disease characterized by bladder calculi (cystine has low solubility). Patients with cystine stones face recurrent urolithiasis and repeated urinary infections.

Normal Findings ⬆ ⬇

Random specimen: Negative

24-hour specimen, adult: <250 mg/L

Values are age dependent; that is, they decrease throughout life.

Procedure ⬆ ⬇

  1. Obtain a random 20-mL urine specimen for a qualitative screening test. Label the specimen with the patient’s name, date and time of collection, and test(s) ordered.

  2. When collecting a 24-hour urine specimen, the container needs a preservative (toluene). Follow general procedures for a 24-hour urine specimen (see Long-Term, Timed Urine Specimen [2-Hour, 24-Hour]).

Clinical Implications ⬆ ⬇

  1. Urine cystine is increased in cystinuria (up to 20 times normal).

  2. Urine cystine is decreased in burn patients.

Clinical Alert

  1. Cystinosis, a different entity from cystinuria, is not detected by cystine studies. Most patients with infantile nephropathic cystinosis have neurologic defects that become apparent in infancy. Failure to thrive and kidney dysfunction are evidence of this disease.

  2. Patients with cystinosis have a defect in renal tubular reabsorption that develops into Fanconi syndrome, which leads to a generalized aminoaciduria. Cystine is elevated in the urine in the same proportion as all amino acids; the concentration is not high enough to form cystine stones. Plasma cystine is normal, but cystine is elevated in kidneys, eyes, spleen, and bone marrow; for purposes of diagnosis, it is usually measured in WBCs.

Interventions ⬆

Pretest Patient Care

  1. Explain purpose of test, procedure for timed urine collection, and interfering factors.

  2. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

Posttest Patient Care

  1. Review test results; report and record findings. Modify the nursing care plan as needed. Counsel patient regarding abnormal findings.

  2. Follow guidelines in Chapter 1 for safe, effective, informed posttest care.