Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (886 page)

   Limitations

   The results of a genetic test may be affected by DNA rearrangements, blood transfusion, bone marrow transplantation, or rare sequence variations.

CYSTINE, URINE (CYSTINURIA PANEL)

   Definition

   Cystinuria is an autosomal recessive defect in reabsorptive transport of cystine and the dibasic amino acids ornithine, arginine, and lysine from the luminal fluid of the renal proximal tubule and small intestine. The only phenotypic manifestation of cystinuria is cystine urolithiasis, which often recurs throughout an affected individual’s lifetime.

   The disorder is divided into three subtypes: Rosenberg I, II, and III. Cystinuria type I is the most common variant. Type I heterozygotes show normal aminoaciduria. Heterozygotes of types II and III often manifest cystinuria without cystine calculi and may be at increased risk for other types of urolithiasis. Type I heterozygotes are distinguished by normal levels of urinary cystine.

   Unlike type I and type II homozygotes, type III homozygotes show an increase in plasma cystine concentration after oral cystine administration.

   To classify cystinuria clinically, urinary cystine can be measured in each parent of a proband as phenotype I (recessive, urinary cystine level <100 μmol/g of creatinine), phenotype II (dominant, urinary cystine level >1,000 μmol/g of creatinine), and phenotype III (partially dominant, urinary cystine level 100–1,000 μmol/g of creatinine). Cystinuria can also be classified based on the age at which symptoms first appear (i.e., infantile, juvenile, adolescent).

   
Normal range:
see Table 16.27.

TABLE 16–27. Age-Based Reference Range for Cystine, Arginine, Lysine, and Ornithine

   Use

   Diagnosis of cystinuria

   Monitoring of patients with cystinuria on therapy

   Interpretation