Comparison of Plasma GFAP and UCH-L1 Levels Measured by Abbott Alinity i and Quanterix Simoa Assays in Traumatic Brain Injury Subjects: A TRACK-TBI Study

Variability in glial fibrillary acidic protein (GFAP) and ubiquitin carboxy-terminal hydrolase L1 (UCH-L1) values across different assay platforms complicates cross-study comparisons and pooled analyses. We compared GFAP and UCH-L1 concentrations analyzed using the Abbott Alinity i TBI test (Alinity) and the Quanterix Neuro Four-plex B assay (Quanterix). We analyzed paired plasma samples from patients enrolled in the Transforming Research and Clinical Knowledge in Traumatic Brain Injury study. GFAP was measured in 185 participants, and UCH-L1 in 181. We assessed the correlation between the two assays using Spearman’s correlation coefficient and agreement using the Bland–Altman plot. We applied Passing–Bablok regression to generate the conversion equations from Alinity to Quanterix. The Quanterix platform reported higher GFAP concentrations (median = 2229 pg/mL) than the Alinity platform (median = 423 pg/mL), with a strong correlation (ρ = 0.992). Passing–Bablok regression comparing the full range of Quanterix versus Alinity on log-transformed data produced a slope of 1.05 (95% confidence interval [CI]: 1.03–1.07) and an intercept of 0.64 (95% CI: 0.59–0.69), indicating a systematic difference across the range. For UCH-L1, the Alinity platform yielded higher values (median = 379 pg/mL) compared with Quanterix (median = 128 pg/mL), with a moderate correlation (ρ = 0.724). The regression slope for the full range log-transformed values was 0.74 (95% CI: 0.65 to 0.84) with an intercept of 0.22 (95% CI: −0.04 to −0.45), indicating a significant proportional difference between the two platforms. GFAP measurements were closely aligned between platforms, while the UCH-L1 values had more variability. Findings support the feasibility of cross-platform conversion.