Sociodemographic Bias in Large Language Model Clinical Trial Screening

Large language models (LLMs) are increasingly used in randomized clinical trial (RCT) screening, but their potential for sociodemographic bias remains unclear.

To determine whether LLM-based trial screening judgments vary with patient sociodemographic characteristics when clinical details and eligibility criteria are held constant.

Cross-sectional evaluation of Phase II-III RCT protocols from ClinicalTrials.gov (U.S. adult populations; 2023-2024). For each protocol, we created 15 physician-validated clinical vignettes rendered in 34 versions: one control (no identifiers) and 33 identity variants spanning gender, race/ethnicity, socioeconomic status, homelessness, unemployment, and sexual orientation.

Identity labels applied to otherwise identical vignettes, evaluated by nine contemporary LLMs.

Primary: eligibility domain score (1-5 Likert scale) comparing identity variants versus control. Secondary: adherence, resources, risk-benefit, and trust/attitude domains. Mixed-effects models estimated adjusted mean differences with multiplicity-corrected P values; differences <.10 considered trivial.

Of 69 protocols, 58 met inclusion criteria. Analysis of 5,324,400 model evaluations showed eligibility judgments were largely stable: most identity-related differences fell within ±0.05 (transgender woman -.008 [95% CI -.04 to .02]; White male .036 [.01 to .07]). Only homelessness exceeded the trivial threshold (-.121 [-.15 to -.09], P<.001). Secondary domains revealed socioeconomic gradients, particularly for adherence (homeless -.595, P<.001) and resources (homeless -.715, P<.001), with smaller trust/attitude effects and negligible risk-benefit differences.

Bias in LLM-assisted trial screening is conditional. Within fixed criteria, models reason consistently; outside them, they echo the inequities of their data. Responsible deployment in clinical research depends on preserving that boundary so that automation strengthens fairness in trial access rather than inheriting distortion.