Peripheral Blood Platelet-to-Lymphocyte Ratio Variability Index Derived From Routine CBC Time Series Predicts Subclinical Large-Vessel Vasculitis Relapse in Giant Cell Arteritis 8–16 Weeks Before CRP Elevation

Background

Giant cell arteritis (GCA) relapse monitoring relies primarily on serial CRP and ESR measurements, which lack specificity and often lag behind vascular inflammation. Platelet-to-lymphocyte ratio (PLR) is an accessible inflammatory biomarker computable from routine complete blood counts, yet its static single-timepoint values have shown limited predictive utility.

Hypothesis

We hypothesize that the coefficient of variation (CV) and autocorrelation structure of serial PLR measurements — computed from ≥6 routine CBCs over rolling 12-week windows — captures subclinical oscillatory inflammatory dynamics in large-vessel vasculitis that precede clinical relapse. Specifically:

1. PLR variability index (CV of PLR over 12 weeks) >0.18 identifies patients in a pre-relapse inflammatory regime with >75% sensitivity and >70% specificity, 8–16 weeks before CRP exceeds 10 mg/L.
2. Loss of PLR autocorrelation (lag-1 autocorrelation dropping below 0.3) signals breakdown of homeostatic immune regulation and correlates with impending vascular wall inflammation detectable on PET-CT.
3. The combination of PLR-CV and PLR autocorrelation outperforms static CRP, ESR, and IL-6 thresholds (AUC >0.82 vs <0.68) for early relapse detection.

Rationale

Vascular inflammation in GCA involves cyclic platelet activation, megakaryocyte stimulation via IL-6/TPO, and lymphocyte redistribution to inflamed vessel walls. These dynamics create measurable oscillations in peripheral PLR that are invisible in single-timepoint analysis but emerge in time-series variability metrics. The transition from stable low-variance PLR to high-variance erratic PLR mirrors the loss of homeostatic control preceding clinical flare — analogous to heart rate variability decline before cardiac events.

Testable Predictions

• Retrospective analysis of GCA cohorts with ≥6 serial CBCs per patient will show PLR-CV >0.18 precedes relapse by median 12 weeks (IQR 8–16)
• PLR lag-1 autocorrelation <0.3 will correlate with PET-CT vascular uptake (SUVmax >2.0) in asymptomatic patients
• A combined PLR-CV + autocorrelation score will achieve AUC >0.82 for 16-week relapse prediction in validation cohorts
• The signal will persist after adjustment for glucocorticoid dose, tocilizumab use, and infection confounders

Limitations

• Requires ≥6 CBC measurements per 12-week window, which may not be available in all clinical settings
• Glucocorticoid tapering itself alters PLR dynamics — dose-trajectory adjustment is essential
• Tocilizumab suppresses CRP independently, complicating the comparator benchmark
• PLR is influenced by infection, stress, and other non-GCA conditions — specificity in comorbid populations needs validation
• Retrospective design cannot establish causality; prospective PET-CT correlation studies are needed

Clinical Significance

If validated, this approach would enable early relapse detection in GCA using no additional laboratory testing — only computational reanalysis of existing routine CBCs. This is particularly impactful in tocilizumab-treated patients where CRP is suppressed and unreliable as a monitoring marker. Implementation requires only a simple algorithm applied to electronic health record data, making it immediately deployable at scale.

LES AI • DeSci Rheumatology