PFAS co-positivities identified in more than 10,000 serum/plasma samples

Per- and polyfluoroalkyl substances (PFAS) are environmentally persistent chemicals to which nearly all humans have detectable exposure. Although clinical interpretation of PFAS biomonitoring data often focuses on individual analytes, emerging toxicological evidence indicates that PFAS mixtures may exert additive or synergistic effects relevant to health outcomes. To characterize PFAS co-positivity patterns, we evaluated PFAS combinations in 10,566 human serum/plasma samples analyzed by targeted liquid chromatography-tandem mass spectrometry (LC-MS/MS) between December 2023 and December 2024. Results from two different PFAS panels offered by the testing laboratory were evaluated: a 13-analyte panel (Panel 1, n = 10,478) and an 18-analyte panel (Panel 2, n = 88). Overall, 98.8% of samples (n = 10,436) contained at least one PFAS. Only 19 samples (0.18%) contained a single PFAS above the lower limit of quantification (0.1 ng/mL). In contrast, most samples showed complex mixtures: 58 unique PFAS combinations were identified in Panel 1 and 16 in Panel 2. Across both panels, the most common combination, PFHxS, linear PFOA, linear and branched PFOS, PFNA, and PFHpS, was detected in 2,754 samples (26.1%). Among the additional PFAS included in Panel 2, 9CI-PF3ONS, HFPO-DA, and PFOSA were not detected in any sample. ADONA was not detected in either panel. These findings reinforce that PFAS exposure in the U.S. population rarely occurs as isolated compounds. Instead, individuals typically carry body burdens comprising five or more PFAS with differing bioaccumulation properties and half-lives. The high prevalence and consistency of specific PFAS combinations highlight the importance of mixture-based interpretation in biomonitoring, particularly given PFAS' potential endocrine-disrupting activity and the possibility of additive or synergistic biological effects. This large dataset provides a practical reference for identifying prevalent PFAS co-positivity patterns and can inform the design of experimental mixture studies, risk-assessment strategies, and clinical guidance that better reflect PFAS chemical exposures. Continued evaluation of analytical scopes will be essential for characterizing PFAS mixtures as new analogs emerge.