MPO and proteinase 3 (neutrophil products derived from azurophilic granules) had higher plasma concentrations in patients with hyperferritinemia, while NGAL (a neutrophil protein derived from secondary granules) did not; notably, neutrophil counts did not differ between groups stratified according to ferritin levels, suggesting that hyperferritinemia may be a marker of neutrophil activation relating to azurophilic granule release. Here, MPO is linked to isolated hyperferritinemia.