Post-COVID Syndrome in Patients with Chronic Diseases: Clinical Factors Associated with Its Presence in a Real-World Clinical Cohort

Background/Objectives: Post-COVID syndrome (PCS) remains a clinically heterogeneous condition. In patients with chronic diseases, real-world evidence is needed to describe its frequency and to identify clinical factors associated with its current presence. This study assessed the prevalence of PCS and factors associated with current PCS status among patients with chronic diseases. Methods: This observational cross-sectional study included a consecutively enrolled source clinical sample of 850 adults receiving inpatient or outpatient care in Almaty and the Almaty Region between June and December 2025. The primary analytic cohort comprised participants with documented prior COVID-19 (n = 250). PCS was assessed at study enrollment according to the national clinical protocol as current symptoms persisting for more than 12 weeks and not explained by an alternative diagnosis. Multivariable logistic regression was used to estimate adjusted odds ratios (ORs) and 95% confidence intervals (CIs). COVID-19 vaccination was excluded from the primary model and evaluated only in a sensitivity analysis because vaccination timing relative to infection was unavailable. The exploratory association-based probability model was evaluated using the AUC with 95% CI, Brier score, calibration assessment, bootstrap internal validation with 1000 resamples, assessment of age nonlinearity, and decision curve analysis. Results: PCS was identified in 147 of 250 participants with prior COVID-19 (58.8%). In the revised primary multivariable model, age (OR = 1.24 per 10-year increase; 95% CI: 1.01–1.54; p = 0.040) and endocrine diseases (OR = 3.36; 95% CI: 1.69–6.98; p < 0.001) remained associated with current PCS status. The association with endocrine diseases persisted after adjustment for body mass index. The apparent AUC was 0.687 (95% CI: 0.620–0.753), and the Brier score was 0.218. Bootstrap internal validation yielded an optimism-corrected AUC of 0.655, a calibration intercept of 0.061, and a calibration slope of 0.816. Conclusions: PCS was frequent among patients with chronic diseases who had prior COVID-19. Older age and endocrine pathology were the most consistent factors associated with current PCS status. All model-performance estimates were obtained within the development sample; the model should not be used for individual prediction or follow-up planning without prospective external validation.