The Contingency of Reported sST2 Serum Concentrations with a Protein Detection System (ELISA) from the Same Manufacturer (R&D Biotechne, 2002–2025): An Explanatory Effort by Applied Medical Researchers

<b>Background/Objectives</b>: Soluble ST2 (sST2) has gained recognition as a clinically relevant biomarker across a spectrum of inflammatory, cardiovascular, and respiratory conditions. However, the lack of assay standardization raises concerns about result comparability across platforms and studies. <b>Methods</b>: This study systematically evaluated serum sST2 concentrations measured with two ELISA systems-DuoSet and Quantikine-produced by the same manufacturer (R&D Systems, Minneapolis, MN, USA). <b>Results</b>: Using archived serum samples from healthy volunteers and marathon runners, we identified marked discrepancies: serum sST2 concentrations using the DuoSet recombinant standard were on average 4.3-fold higher than those using Quantikine (median 308.3 [106.6-608.6] vs. 71.5 [41.8-115.6] ng/mL). On the pre-coated Quantikine plate, using the DuoSet recombinant standard increased calculated concentrations 4.3-fold compared with the native Quantikine standard (median 308.3 [106.6-608.6] vs. 71.5 [41.8-115.6] ng/mL). On the manually coated DuoSet plate, the DuoSet standard yielded higher medians than the Quantikine standard (8.0 [5.6-11.3] vs. 5.0 [3.7-7.4] ng/mL). Furthermore, between-lot variability within the same ELISA platform resulted in concentration shifts from 0.09 [0.07-0.10] ng/mL (2016) to 1.17 [0.81-3.23] ng/mL (2023) using the same sample. Previously published studies also exhibited wide inter-study variability among healthy cohorts. <b>Conclusions</b>: These findings emphasize that current ELISA systems for sST2 are not standardized and that cross-study comparisons should be interpreted with caution. Until universal standardization is implemented, sST2 should primarily be used for within-study comparisons. This variability may limit the reliability of longitudinal sST2 assessment even in clinical settings.