Cystine crystals indicate an abnormality in the metabolism of the amino acid cysteine. These crystals appear as colorless, refractile, hexagonal plates with even sides. Cystine crystals sometimes occur in pairs. They produce a red color in the nitroprusside reaction. Cystine may be confused with hexagonal uric acid forms, but it does not polarize light. Cystine crystals occur in acidic urine.
