Erythrocyte-Associated Proteins Underlie the Reported Oxidative-Stress Enrichment in Plasma Proteomics of Obstructive Sleep Apnoea: A Secondary Quality Marker Analysis of Public Datasets

Background: Erythrocyte proteins released into plasma carry oxidation-related annotations and can be read as evidence of oxidative stress. We examined whether this applies to an oxidative-stress enrichment reported in a plasma proteomic study of obstructive sleep apnoea (OSA). Methods: In a secondary analysis of public data (PXD073409; men; 15 non-OSA, 15 mild OSA, 15 severe OSA; 422 proteins), erythrocyte, platelet and coagulation quality marker indices were computed from the panels of Geyer et al.; the erythrocyte panel was only partly quantified (15 of 29 entries). The original differential and over-representation analyses were repeated with panel members excluded, with adjustment for the erythrocyte index, and with leave-one-protein-out indices. A second public dataset (PXD069503; 15 severe OSA, 15 mild-to-moderate OSA, 15 controls) was used for directional comparison. Clinical covariates were not available. Results: The erythrocyte index was higher in mild (ratio of geometric means 1.74, 95% CI 1.04 to 2.80) and severe OSA (2.43, 1.68 to 3.47) than in non-OSA participants. Six of the 13 proteins that are increased in severe OSA were erythrocyte panel members, and all nine enriched terms consisted of panel members only, including hydrogen peroxide catabolic process (HBA1, HBB, HBD, PRDX2). After adjustment, PRDX2 was accounted for by erythrocyte content, whereas the haemoglobin chains remained elevated and carried seven of the terms. The index alone separated severe- from non-OSA participants (AUROC 0.858); the data could not show whether all 422 proteins (AUROC 0.969) add to it (difference +0.111, 95% CI −0.009 to +0.268). In the second dataset the index was higher in severe OSA than in controls (1.55, 1.06 to 2.35). Conclusions: The reported oxidative-stress enrichment rests on erythrocyte-associated proteins and, on these data, is not evidence of altered oxidative activity in plasma. Whether this content arises before or after venepuncture cannot be determined. Quality marker indices should be reported alongside plasma proteomic biomarker results.