Introduction

Post-COVID-19 syndrome (PCS) is a complex multisystem condition marked by prolonged symptoms after acute SARS-CoV-2 infection. This study investigated clinical, biochemical, and gut microbiota alterations in PCS patients (n = 28) before and after a structured 14-day rehabilitation program, compared to healthy controls (n = 48).

Materials and Methods
Comprehensive evaluations included the following: hematological, coagulation, and biochemical profiling using a UniCel DxH800 hematology analyzer (Beckman Coulter, USA) and Cobas e411 system (Roche, Switzerland); serum metabolite analysis via gas chromatography–mass spectrometry (Trace GC 1310 and ISQ LT; Thermo Electron Corporation, USA); and gut microbiota assessment with real-time PCR (CFX 96, BioRad, USA) using Colonoflor-16 kits (AlphaLab, Russia) to quantify bacterial taxa.
Data were compared against reference ranges and age-matched controls.

Results
Post-rehabilitation, patients reported subjective improvements in respiratory function, mood, and overall well-being. However, persistent abnormalities included the following. Elevated metabolic markers: 4-hydroxybenzoic acid, succinic acid, and fumaric acid. Pro-inflammatory mediators: interleukin-6 levels remained 2.1× higher than controls. Microbiota dysbiosis: increased total bacterial load (4.8 log10 CFU/g), reduced Lactobacillus spp., and higher pro-inflammatory taxa. Systemic inflammation: 25% of patients showed elevated ESR (>20 mm/h), and 20% had elevated CRP (>5 mg/L).

Conclusion
While rehabilitation alleviated clinical symptoms, residual metabolic dysregulation and inflammation suggest incomplete recovery. ESR and CRP levels may serve as practical biomarkers for monitoring post-COVID-19 interventions. Acute-phase reactants and microbiota profiles could guide personalized management strategies in PCS.