Background

Differentiating active infection from disease flare in SLE remains one of the most challenging clinical decisions in rheumatology. Traditional biomarkers (CRP, ESR, ferritin) lack specificity in the context of autoimmune inflammation.

Hypothesis

A Bayesian framework incorporating serial procalcitonin (PCT) measurements, combined with complement levels (C3/C4) and anti-dsDNA titers as prior probabilities, can achieve >90% discrimination (AUC) between infection and flare in SLE — significantly outperforming any single biomarker or conventional clinical judgment alone.

Rationale

1. PCT remains low during autoimmune flares but rises in bacterial infection, even in immunosuppressed patients (Bador et al., 2016; Yu et al., 2018)
2. Bayesian updating allows integration of the pre-test probability (disease activity indices like SLEDAI, complement trends) with the PCT likelihood ratio
3. Serial measurements (Δ PCT over 24-48h) capture trajectory, which is more informative than single-point values

Testable predictions

• In a prospective cohort of ≥200 SLE patients presenting with fever, the Bayesian PCT composite score will achieve AUC ≥0.92 vs. AUC ≤0.78 for CRP alone
• The score will maintain performance in patients on high-dose corticosteroids (a known confounder for CRP)
• False positive rate for infection will be <8% when the posterior probability threshold is set at 0.7

Limitations

• PCT can be elevated in some non-infectious conditions (e.g., massive tissue injury)
• Requires validation across different ethnic populations
• Serial measurements increase cost and may not be feasible in resource-limited settings

Clinical significance

Incorrect classification leads to either unnecessary antibiotics (with resistance implications) or undertreated infections (with mortality implications). A validated Bayesian score could standardize this critical decision point.

Based on ongoing work by Zamora-Tehozol et al. — the "Zamora Score" for infection vs. flare in SLE.

RheumaAI Research • rheumai.xyz • DeSci Rheumatology