1/2 Cystine (plasma amino acid)

The 1/2 Cystine test measures the oxidized form of cysteine, reported as half-cystine, in your blood. Cysteine and cystine are sulfur-containing amino acids that help build proteins and support the body’s antioxidant defenses through pathways that include glutathione. Because cysteine readily oxidizes to cystine outside the body, many laboratories report this component as half-cystine on plasma amino acid profiles.

This test is usually performed as part of a quantitative plasma amino acid analysis. It helps your care team assess amino acid balance in the context of nutrition, metabolism, and certain inherited conditions that affect how the body handles sulfur-containing amino acids.

Results can provide clues about nutritional status, adequacy of parenteral nutrition, and disruptions in sulfur amino acid metabolism. Abnormal values may be seen with malabsorption, liver or kidney disease, or during significant oxidative stress or inflammation. The measurement can also contribute to the evaluation of metabolic disorders, alongside other plasma and urine amino acids, organic acids, and targeted genetic tests.

Clinicians may order this test when investigating unexplained developmental concerns, poor growth, muscle weakness, unusual odors, or recurrent metabolic decompensation, or when monitoring specialized nutrition. While disorders like cystinuria and cystinosis involve cystine, they often require specific urine studies or leukocyte testing for diagnosis and monitoring, so this plasma test is interpreted together with other results.

Your result is interpreted in the context of age-specific expectations and your overall clinical picture. A value that is slightly outside the expected interval may be due to sample handling, diet, or timing, and your clinician may repeat the test or review other amino acids to confirm a pattern.

If results are higher or lower than expected, your provider may look for causes such as diet or supplement use, medication effects, oxidative stress, or organ function issues. Follow-up could include a full amino acid profile, urine amino acids, nutritional assessment, or genetic testing if an inherited condition is suspected. Do not change medications or supplements without discussing them with your care team.