GlycA is the name given to a particular inflammation-responsive signal in proton (
1H) nuclear magnetic resonance (NMR) spectra of serum and plasma measured clinically [
1]. The GlycA NMR signal arises largely from the
N-acetyl methyl group protons of mobile
N-acetyl glucosamine residues on the glycan portions of acute-phase proteins such as α1-acid glycoprotein (orosomucoid), haptoglobin, α1-antitrypsin and α1-antichymotrypsin [
1‐
3]. GlycA levels, in units of μmol/L glycoprotein
N-acetyl methyl groups, are associated with concentrations of IL-6, tumor necrosis factor (TNF)-α, fibrinogen, C-reactive protein (CRP; hsCRP as assessed by high-sensitivity assay), serum amyloid A (SAA), and lipoprotein-associated phospholipase A
2 (Lp-PLA
2) [
1,
3‐
9]. Moreover, GlycA levels are associated with increased production of anti-microbial peptides, circulating leukocytes and neutrophil activity [
3,
10]. In fact, two of the major protein contributors to the GlycA signal, α1-acid glycoprotein and haptoglobin, are synthesized in and secreted from neutrophil granules, suggesting that, besides the liver, neutrophils may be a relevant source of elevated GlycA [
3,
10]. However, CRP, SAA and cytokines circulate at much lower concentrations than α1-acid glycoprotein, haptoglobin, and α1-antitrypsin, and are not highly glycosylated, therefore they contribute negligibly to the measured GlycA signal [
1]. Reduced glycan mobility is another reason why some proteins with
N-acetyl glucosamine residues, such as fibrinogen and immunoglobulin G (IgG), do not produce observable glycan NMR signals [
1]. Since both positive acute phase protein levels and glycan complexity increase during inflammation, GlycA is higher in patients with acute febrile illnesses as well as chronic inflammatory diseases [
3,
10,
11].