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The diagnosis of cystathionine β-synthase (CBS) deficiency and disorders of methionine remethylation depend on the demonstration of homocyst(e)inemia and homocystinuria. To be detectable in the usual analysis of amino acids, homocystine must remain as a free disulfide in the plasma or serum. However, homocystine undergoes an exchange reaction with free sulfhydryls, including cysteine, readily forming covalent disulfide bonds with these thiols, and gradually disappears as a free amino acid (1). Because preparation for quantitative amino acid analysis usually includes deproteinization, protein-bound homocysteine is not detected by standard methods (2)(3)(4). Homocysteine-cysteine mixed disulfide, also increased in the homocystinurias, undergoes a similar exchange.This phenomenon is still not appreciated by many laboratories that perform amino acid analyses, and lack of awareness of the phenomenon continues to result in missing the diagnosis of homocystinuria. This was brought to our attention by the case of an infant with CBS-deficient homocystinuria. Routine newborn screening had identified an increased concentration of blood methionine (5). Quantitative plasma amino acid analysis 1 week later disclosed isolated hypermethioninemia. …

Inadequate Laboratory Technique for Amino Acid Analysis Resulting in Missed Diagnoses of Homocystinuria