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Letters |
Institute and Policlinic of Clinical Metabolic Research, Medical Faculty Carl Gustav Carus, Technical University Dresden, Fetscherstrasse 74, D-01307 Dresden, Germany
a Author for correspondence. Fax 49-351-458-5306; e-mail julius{at}urz.rcs.tu-dresden.de.
To the Editor:
The determination of total homocysteine (tHcy) for the diagnosis and therapy of folate and cobalamin (vitamin B12) deficiencies has become an important feature of the clinical chemistry laboratory. In addition, because of the potential use of tHcy as an independent risk factor for cardiovascular disease and thromboembolism, the establishment of accurate and reliable tHcy assays has gained importance. As a result, a number of different analytical protocols to measure tHcy concentrations in human serum or plasma, using HPLC and gas chromatography/mass spectrometry (GC/MS) techniques have been described (1). A recent article by Frantzen et al. (2) described an enzyme conversion immunoassay (EIA) for measurement of tHcy in plasma or serum. This assay is based on enzymatic conversion of tHcy (after reduction and release of endogenous homocysteine from proteins and/or disulfides) to S-adenosyl-L-homocysteine (SAH) by the action of SAH hydrolase (EC 3.3.1.1), followed by quantification of SAH in a competitive immunoassay with use of a monoclonal antibody against SAH. Frantzen et al. demonstrated a good method quality and showed their results obtained by the new assay to be well-correlated with a commonly used HPLC method (3) (as cited in (2)). They emphasized that the enzymatic method may serve as an alternative to HPLC analysis in clinical routines and research.
We performed a systematic comparison of the new EIA (available from
Axis Biochemicals ASA, Oslo) and the GC/MS methods published by Stabler
et al. (4) and, more recently, by our group (5).
We determined the concentrations of tHcy by these methods in plasma
samples of 104 volunteers (ages, 20–69 years; 43 females, 61 males;
tHcy range, 1.60–82.34 µmol/L) with good precision (intra- and
interassay CVs were <6.2% and <8.0%, respectively, for the EIA and
<3.5% and <4.8%, respectively, for the GC/MS methods). The EIA and
GC/MS methods agreed well (Table 1
), with results of the GC/MS methods 1% higher than the EIA
results. This may be regarded as good agreement with the enzymatic
method.
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The new EIA has an acceptable precision and analysis range. It is quick and simple to use and can be adopted immediately by any laboratory. It appears to be an excellent choice for most routine laboratory purposes, particularly for monitoring of oral folate, betaine, and/or pyridoxine therapy, as well as for large clinical studies on the role of tHcy as a cardiovascular risk factor. On the other hand, the advantages of the GC/MS methods are, despite their cumbersomeness, their excellent lower limits of quantification (~0.2 µmol/L by GC/MS), their extended analytical range (0.2–300 µmol/L by GC/MS vs 2.0–50 µmol/L by EIA), and the possibility to determine simultaneously other metabolites of homocysteine turnover, e.g., cystathionine (4) and/or the other sulfur-containing amino acids, cysteine and methionine (5). The latter is especially required when performing studies that aim at the response of tHcy concentrations to an oral methionine challenge in several metabolic disorders and diseases (6)(7).
References
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