Study 13 — Cost-Effectiveness of Routine Toxoplasma gondii Serology at First-Episode Psychosis

Study 13 of the Hidden Infections research series. Zero of six major international psychiatric guidelines include Toxoplasma gondii serology in first-episode psychosis (FEP) workup, despite 60 years of evidence linking T. gondii seropositivity to elevated schizophrenia risk (Study 2). This decision analysis models the cost-effectiveness of adding routine T. gondii serology to FEP workup in a hypothetical 10,000-patient cohort. Key findings: Serology is cost-effective (below $50,000/QALY threshold) at treatment response rates >=10% (ICER $63,247/QALY). At >=30% treatment response rate, the strategy is cost-dominant. Sensitivity analyses across seroprevalence (15-40%), serology cost ($5-$50), and quality-of-life assumptions confirm robustness at moderate response rate assumptions. The analysis demonstrates the threshold at which a randomised trial becomes economically justified. Group D: Clinical and economic modelling. CC0 public domain. AI-assisted draft pre-print.