Phosphorylated Tau 217 Varies With Kidney Function Even Without Chronic Kidney Disease

Importance. Plasma phosphorylated tau 217 (p-tau217) is entering clinical use with fixed diagnostic threshold that do not account for kidney function. Chronic kidney disease (defined as estimated glomerular filtration rate (eGFR) below 60 mL/min/1.73 m2), raises plasma p-tau217 concentrations and can produce false positive results. It remains unclear whether milder reductions in kidney function, above the CKD threshold, have the same effect. Objective. To determine whether plasma p-tau217 is elevated with mildly reduced kidney function, above the chronic kidney disease threshold, in adults without neurological disease. Design. Cross-sectional analysis of a cohort enrolled between 2023 and 2025 and biomarker analysis blinded to clinical data. Setting. Two private psychiatric inpatient facilities in Australia. Participants. Adults admitted with psychiatric disorders and free of neurological disease. Exposures. eGFR calculated from serum creatinine using the CKD-EPI 2021 equation. Main Outcomes and Measures. Plasma p-tau217, neurofilament light chain (NfL), and glial fibrillary acidic protein (GFAP), measured on the Simoa platform. Multivariable linear regression compared eGFR groups (< 60, 60 to 90, and [&ge;] 90 mL/min/1.73 m2), adjusted for age, BMI, and sex. Results. Among 180 participants (median age, 50.5 years [IQR, 38.0-61.5]; 94 [52%] female), eGFR was [&ge;] 90 mL/min/1.73 m2 in 106 (59%), 60 to 90 in 61 (34%), and < 60 in 13 (7.2%). Compared with an eGFR [&ge;] 90, plasma p-tau217 was 36% higher in both the 60 to < 90 group ({beta} = 0.42 [95% CI: 0.11, 0.74]; P = .008) and 116% higher in the < 60 group ({beta} = 1.05 [95% CI: 0.43, 1.72]; P = .002). Plasma NfL and GFAP were elevated in the < 60 group but differences in the 60 to < 90 group did not reach significance. Modelled continuously, plasma p-tau217 was 9.5% lower per 10 mL/min/1.73 m2 higher eGFR (95% CI: 3.3%, 15.4%). Conclusions and Relevance. Plasma p-tau217 was elevated in adults with mildly reduced kidney function, above the CKD threshold, in a cohort free of neurological disease. Kidney function across its full range should be incorporated into plasma p-tau217 reference ranges and clinical interpretation to reduce false positive and false negative results.