The Global Arginine Bioavailability Ratio (GABR) test is a blood-based assay designed to assess the bioavailability of arginine relative to its metabolic counterparts, ornithine and citrulline. This test provides insight into endothelial function, nitric oxide synthesis, and cardiovascular health by measuring the balance of these amino acids, which are critical for vascular homeostasis. It is primarily used in research and clinical settings to evaluate endothelial dysfunction, cardiovascular risk, and impaired nitric oxide production conditions, including hypertension, atherosclerosis, and metabolic disorders.
Arginine is a semi-essential amino acid, the sole precursor for nitric oxide (NO), a key signaling molecule in vasodilation, immune function, and neurotransmission. The bioavailability of arginine is influenced by its metabolism through multiple pathways, including conversion to nitric oxide via nitric oxide synthase (NOS) and degradation into ornithine and urea by arginase. The competition between these metabolic pathways determines the availability of arginine for nitric oxide synthesis, measuring its balance with ornithine and citrulline essential for assessing endothelial function. A reduced GABR indicates impaired nitric oxide production and is associated with endothelial dysfunction, oxidative stress, and increased cardiovascular risk.
GABR is calculated by determining the arginine ratio to the sum of ornithine and citrulline concentrations in plasma. This ratio reflects the efficiency of arginine utilization and provides a more comprehensive assessment of nitric oxide bioavailability than absolute arginine levels alone. Given the role of arginine metabolism in various physiological processes, deviations in GABR have been linked to a wide range of pathological conditions, including endothelial dysfunction, hypertension, diabetes, chronic kidney disease, and inflammatory disorders.
The physiological significance of GABR extends beyond cardiovascular health, as arginine metabolism plays a crucial role in immune regulation and metabolic homeostasis. Inflammatory conditions and oxidative stress can alter arginine availability by upregulating arginase activity, reducing nitric oxide production and endothelial dysfunction. This imbalance is observed in metabolic syndrome, diabetes mellitus, and chronic inflammatory diseases, where nitric oxide deficiency contributes to disease progression. Additionally, conditions such as sickle cell disease and pulmonary hypertension have been associated with altered arginine metabolism and reduced bioavailability, further emphasizing the clinical relevance of this biomarker.
The results of this test provide valuable information regarding vascular function and metabolic status, offering a targeted approach for evaluating nitric oxide bioavailability and endothelial health.
