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Background: Low levels of the endogenous amino acid L-homoarginine are a risk factor for cardiovascular morbidity and mortality. For individual risk prediction, commercially available test systems are mandatory. This study aims at formulating sex- and age-specific reference intervals of serum L-homoarginine determined with an ELISA., Methods: We determined reference intervals for serum L-homoarginine stratified by age and sex in a sample of 1285 healthy participants of the Study of Health in Pomerania (SHIP)-TREND cohort after exclusion of participants with cardiovascular diseases, diabetes mellitus, hypertension, metabolic syndrome, elevated liver enzymes, chronic kidney disease stages III or IV, or body mass index >25 kg/m2. Serum L-homoarginine was determined applying a commercially available ELISA., Results: The reference cohort included 836 women (median age 41, 25th and 75th percentiles are 32 and 50 years) and 449 men (median age 38, 25th, and 75th percentiles are 30 and 49 years). The median serum concentration of L-homoarginine was 1.93 (25th 1.49; 75th 2.60) µmol/L in women and 2.02 (25th 1.63; 75th 2.61) µmol/L in men (P = 0.04). The reference intervals (2.5th to 97.5th percentile) were 0.89-5.29 µmol/L for women and 1.09-3.76 µmol/L for men. The L-homoarginine serum concentration declined over age decades in both sexes and a notable interaction with sex hormone intake in women was observed., Conclusions: The novelty of our study is that we determined reference intervals specific for the L-isomer being lower than those previously reported for homoarginine in SHIP and thus might be helpful in identifying individuals suitable for oral L-homoarginine supplementation., Competing Interests: Authors’ Disclosures or Potential Conflicts of Interest: Upon manuscript submission, all authors completed the author disclosure form. Disclosures and/or potential conflicts of interest: Employment or Leadership: R. Wesemann is managing director of WESAMIN GmbH & Co. KG, Winsen, Germany, responsible for the development and production of the DLD Diagnostika ELISA kit for L-homoarginine. Consultant or Advisory Role: None declared. Stock Ownership: None declared. Honoraria: C.-u.Choe received honoraria from Zambon GmbH. Research Funding: This work was funded in part by a grant from the DZHK (Deutsches Zentrum für Herz-Kreislauf-Forschung e.V., E. Schwedhelm, T. Ittermann, N. Friedrich). The SHIP-TREND study is part of the Community Medicine Research net of the University of Greifswald, Germany, which is funded by the Federal Ministry of Education and Research (Grants 01ZZ9603, 01ZZ0103, and 01ZZ0403), the Ministry of Cultural Affairs as well as the Social Ministry of the Federal State of Mecklenburg-West Pomerania (T. Ittermann, N. Friedrich). C.-u.Choe was supported by an Else Kröner Exzellenzstipendium from the Else Kröner-Fresenius Stiftung. Expert Testimony: None declared. Patents: None declared., (© American Association for Clinical Chemistry 2022. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Asymmetric dimethylarginine (ADMA) is an endogenous competitive inhibitor of nitric oxide synthase (NOS). Elevated ADMA plasma levels have been reported in connection with diseases associated with an impaired endothelial L-arginine-NO pathway and endothelial dysfunction, such as atherosclerosis, hypercholesterolemia, chronic heart failure, diabetes mellitus, and hypertension. NO production by NOS is decreased due to elevated ADMA levels. In fact, there is increasing interest in determination of ADMA levels in samples of various origins. The aim of this work was to develop a precise and easy immunoassay in contrast to the existing methods, such as HPLC, liquid chromatography-mass spectrometry (LC-MS) and gas chromatography (GC)-MS. We determined cross-reactivity in our immunoassay of 1.2% for symmetric dimethylarginine and <0.02% for L-arginine. The limit of quantitation was 0.05 micromol/l. We found good correlation of the values measured when we compared our assay with LC-tandem MS (n = 29; r = 0.984; p < 0.0001). We determined ADMA levels in human serum and plasma, mouse and rat plasma, and cell culture supernatant. For human plasma we found a mean of 0.65 micromol/l in healthy subjects. In the plasma of mice and rats we found mean concentrations of 1.05 and 1.09 micromol/l, respectively.

Determination of urinary 3-O-methylated catecholamines (metanephrines) is generally considered a principal test for the clinical chemical diagnosis of pheochromocytoma and is currently performed predominantly with chromatographic techniques such as gas-liquid chromatography and HPLC. Enzyme immunoassays based on microtiter plate technology have recently been developed for the quantitative determination of urinary metanephrine (M) and normetanephrine (NM). We compared the results for urinary M and NM determined by these ELISA methods with those obtained by a recently developed isotope dilution mass spectrometric method. From this comparative study we can conclude that the investigated ELISA methods are applicable in the quantification of urinary M and thus can be successfully used to establish the diagnosis of pheochromocytoma. These relatively simple methods can be executed in any clinical laboratory and in time may replace the present, more complicated, chromatographic techniques.