European Journal of Cardiovascular Medicine

Inflammation is a fundamental biological response of the body to harmful stimuli, including pathogens, tissue injury, and metabolic stress. The inflammatory cascade involves a complex interplay of cellular and molecular events, orchestrated by pro-inflammatory cytokines and acute-phase proteins. Among the myriad biomarkers that have been identified to monitor this response, C-reactive protein (CRP), interleukin-6 (IL-6), and ferritin stand out as the most extensively studied and clinically utilized markers. Each of these biomarkers provides unique insights into the nature, intensity, and duration of the inflammatory process, yet their biochemical interpretation demands a nuanced understanding of their physiological roles and pathophysiological contexts.

C-reactive protein is a pentraxin family protein synthesized primarily by hepatocytes in response to IL-6 stimulation. It is a classic positive acute-phase reactant whose serum concentration can increase 100- to 1000-fold during acute inflammation. CRP plays a critical role in the innate immune response by promoting phagocytosis and activating the complement system. Its rapid rise (within 4–6 hours) and relatively short half-life make it an excellent marker for acute inflammation and a valuable tool for monitoring treatment response. However, CRP is nonspecific and can be elevated in a wide range of conditions, including infections, autoimmune diseases, trauma, and malignancy.

Interleukin-6 is a pleiotropic cytokine that serves as the primary mediator of the acute-phase response. Produced by macrophages, lymphocytes, endothelial cells, and other cell types, IL-6 orchestrates the hepatic synthesis of acute-phase proteins, including CRP, serum amyloid A, and fibrinogen. IL-6 also plays central roles in B-cell differentiation, T-cell activation, and the regulation of the pro-inflammatory versus anti-inflammatory balance. Its elevation precedes that of CRP, making it an earlier marker of inflammation. However, IL-6 has a short half-life and is more technically challenging to measure, limiting its routine clinical use compared to CRP.

Ferritin, traditionally recognized as the primary intracellular iron storage protein, has emerged as a dual-function molecule with significant roles in both iron homeostasis and the inflammatory response. During inflammation, ferritin acts as a positive acute-phase reactant, with serum concentrations increasing in response to IL-6 and other pro-inflammatory cytokines. The IL-6 → hepcidin → ferritin pathway explains why ferritin can rise independently of actual iron stores. This acute-phase behavior complicates the interpretation of ferritin as a marker of iron status, as elevated levels may reflect either iron overload or inflammatory hyperferritinemia. Indeed, the dual role of ferritin as both an iron biomarker and an inflammatory marker has been increasingly recognized.

The distinct biochemical profiles of these markers across different conditions have significant clinical implications. Slaats et al. (2016) proposed a model in which the inflammatory response to bacterial and viral infections presents with specific plasma patterns: bacterial infections are characterized by an IL-6/CRP-dominant response, while viral infections demonstrate an IL-18/ferritin-dominant pattern. This distinction reflects the differential activation of the NLRP3 inflammasome and other pathogen-sensing pathways.

Despite their widespread use, the interpretation of these inflammatory markers remains challenging due to overlapping elevations in various conditions, the influence of non-inflammatory factors, and the lack of standardized reference ranges across different populations and clinical settings. This study aimed to evaluate the biochemical profiles, clinical utility, and interpretative challenges of CRP, IL-6, and ferritin across a spectrum of inflammatory conditions, and to develop a framework for their integrated interpretation in clinical practice.

Study Design and Population

This cross-sectional study was conducted at a tertiary care teaching hospital between January 2016 and December 2018. A total of 450 patients with various inflammatory conditions and 100 age- and sex-matched healthy controls were enrolled. Patients were categorized into four groups based on clinical diagnosis: bacterial infections (n=120), viral infections (n=100), autoimmune diseases (n=130), and chronic inflammatory disorders (n=100).

Inclusion criteria for patients were: (1) age ≥18 years, (2) confirmed clinical diagnosis of the respective condition based on established criteria, (3) active inflammation at the time of enrollment (defined by clinical signs and symptoms), and (4) provision of written informed consent. Exclusion criteria included: (1) pregnancy or lactation, (2) active malignancy, (3) concurrent infections or inflammatory conditions, (4) recent surgery or trauma within the preceding 4 weeks, (5) chronic kidney disease stage 4 or 5, and (6) use of immunosuppressive therapy within the preceding 3 months.

Healthy controls were recruited from individuals undergoing routine health check-ups with no history of acute or chronic inflammatory conditions, infections, or autoimmune diseases. They were matched to the patient groups for age (±5 years) and sex. All participants provided written informed consent, and the study was approved by the institutional ethics committee in accordance with the Declaration of Helsinki.

The study cohort comprised 238 males (52.9%) and 212 females (47.1%), with a mean age of 49.6 ± 15.8 years. Among the autoimmune disease group, diagnoses included rheumatoid arthritis (n=48), systemic lupus erythematosus (n=32), inflammatory bowel disease (n=28), and others (n=22). The chronic inflammatory group included chronic obstructive pulmonary disease (n=40), chronic liver disease (n=32), and chronic kidney disease (n=28).

Biochemical Measurements

Fasting blood samples (10 mL) were collected from all participants between 8:00 and 10:00 AM after an overnight fast of 10–12 hours. Samples were centrifuged at 3,000 rpm for 15 minutes within 2 hours of collection, and serum was aliquoted and stored at -80°C until analysis. All measurements were performed in a centralized, accredited laboratory.

C-Reactive Protein (CRP): Serum CRP levels were measured using a high-sensitivity immunoturbidimetric method (Roche Diagnostics, Basel, Switzerland). The assay has a detection limit of 0.1 mg/L, with intra- and inter-assay coefficients of variation <4% and <6%, respectively. Results were expressed as mg/L.

Interleukin-6 (IL-6): Serum IL-6 levels were measured using a high-sensitivity enzyme-linked immunosorbent assay (ELISA) kit (R&D Systems, Minneapolis, MN, USA). The assay has a detection limit of 0.16 pg/mL, with intra- and inter-assay coefficients of variation <8%. Results were expressed as pg/mL.

Ferritin: Serum ferritin levels were measured using a chemiluminescent immunoassay (Beckman Coulter, Brea, CA, USA). The assay has a detection range of 0.5–2000 ng/mL, with intra- and inter-assay coefficients of variation <5% and <8%, respectively. Results were expressed as ng/mL.

Ferritin/CRP Ratio: The ferritin/CRP ratio was calculated as serum ferritin (ng/mL) divided by serum CRP (mg/L), as previously described.

Clinical Assessment

Disease severity was assessed using validated clinical scoring systems appropriate for each condition: the Acute Physiology and Chronic Health Evaluation (APACHE) II score for infections, the Disease Activity Score 28 (DAS28) for rheumatoid arthritis, the Systemic Lupus Erythematosus Disease Activity Index (SLEDAI) for SLE, and the Crohn's Disease Activity Index (CDAI) for inflammatory bowel disease.

Statistical Analysis

Statistical analyses were performed using SPSS version 22.0 (IBM Corp., Armonk, NY, USA) and MedCalc version 18.0. Continuous variables were expressed as mean ± standard deviation (SD) or median with interquartile range (IQR) for non-normally distributed data. Categorical variables were presented as frequencies and percentages.

Comparisons between groups were performed using one-way ANOVA with Tukey's post-hoc test for normally distributed variables and Kruskal-Wallis test with Dunn's post-hoc test for non-normally distributed variables. Chi-square test was used for categorical variables.

Receiver operating characteristic (ROC) curve analysis was performed to evaluate the diagnostic performance of individual biomarkers and the ferritin/CRP ratio for distinguishing between different inflammatory conditions. Area under the curve (AUC) values were calculated with 95% confidence intervals (CI). Optimal cut-off values were determined using Youden's index.

Correlations between biomarkers and disease severity scores were assessed using Pearson's or Spearman's correlation coefficients as appropriate. A two-sided p-value <0.05 was considered statistically significant

Baseline Characteristics

Table 1 presents the baseline demographic and clinical characteristics of the study population. The four patient groups were comparable in terms of age and sex distribution. As expected, all patient groups had significantly elevated inflammatory markers compared to healthy controls.

Table 1: Baseline Demographic and Clinical Characteristics

*Data presented as mean ± SD or n (%). p<0.001 vs. controls. BMI: body mass index; CRP: C-reactive protein; IL-6: interleukin-6.

Distinct Inflammatory Profiles Across Conditions

Table 2 presents the distinct inflammatory biomarker profiles observed across the different patient groups. Bacterial infections demonstrated the highest CRP and IL-6 levels, with CRP levels exceeding 90 mg/L in most patients. The IL-6/CRP-dominant pattern in bacterial infections is consistent with the classical acute-phase response triggered by bacterial components such as lipopolysaccharide.

In contrast, viral infections demonstrated a distinct pattern with comparatively lower CRP and IL-6 levels but markedly elevated ferritin, consistent with the IL-18/ferritin-dominant pattern previously described. The ferritin/CRP ratio was significantly higher in viral infections (18.6 ± 10.4) compared to bacterial infections (3.2 ± 1.8, p<0.001), reflecting the differential inflammatory programming.

Autoimmune diseases showed moderate elevations of all three markers, with ferritin levels particularly elevated in patients with active disease. Chronic inflammatory disorders demonstrated the lowest elevations, consistent with the more indolent nature of these conditions.

Table 2: Distinct Inflammatory Profiles Across Conditions

*Data presented as mean ± SD. p<0.001 vs. bacterial infections. CRP: C-reactive protein; IL-6: interleukin-6.

Correlations with Disease Severity

Table 3 presents the correlation coefficients between inflammatory markers and disease severity scores. Ferritin demonstrated the strongest correlation with disease severity across all patient groups, particularly in autoimmune diseases (r=0.62, p<0.001) and viral infections (r=0.58, p<0.001). CRP showed strong correlations in bacterial infections (r=0.64, p<0.001), reflecting its role as a sensitive marker of bacterial inflammation. IL-6 demonstrated consistent correlations across all groups, reflecting its role as a proximal mediator of the inflammatory response.

Table 3: Correlation of Inflammatory Markers with Disease Severity

*Data presented as Spearman correlation coefficients (r). p<0.001 for all correlations. CRP: C-reactive protein; IL-6: interleukin-6.

Diagnostic Performance of Ferritin/CRP Ratio

Table 4 presents the diagnostic performance of the ferritin/CRP ratio for distinguishing between different inflammatory conditions. The ferritin/CRP ratio demonstrated excellent discriminatory ability between bacterial and viral infections, with an AUC of 0.88 (95% CI 0.82–0.94). At the optimal cut-off of 8.0, the ratio showed a sensitivity of 84.0% and specificity of 82.5% for identifying viral infections versus bacterial infections.

The ratio also effectively distinguished inflammatory hyperferritinemia from true iron overload. A ratio >15 with elevated ferritin suggested inflammatory hyperferritinemia, while a ratio <5 with elevated ferritin suggested iron overload or tissue stress.

Table 4: Diagnostic Performance of Ferritin/CRP Ratio

AUC: area under the curve; CI: confidence interval; PPV: positive predictive value; NPV: negative predictive value.

Acute-Phase Kinetics

Table 5 illustrates the temporal kinetics of CRP, IL-6, and ferritin during the acute-phase response. CRP levels begin to rise within 4–6 hours of the inflammatory stimulus, peak at 36–50 hours, and decline rapidly with resolution of inflammation. IL-6 elevation precedes that of CRP, with peak levels occurring earlier (12–24 hours) and declining more rapidly due to its short half-life. Ferritin levels rise more slowly, peaking at 48–72 hours, and remain elevated for a longer duration, reflecting its role in the iron sequestration response.

Table 5: Temporal Kinetics of Inflammatory Markers During Acute-Phase Response

Multivariate Predictors of Inflammatory Etiology

Table 6 presents the results of multivariate logistic regression analysis for predictors of inflammatory etiology. Elevated ferritin (OR 3.84, 95% CI 2.24–6.58, p<0.001) and elevated ferritin/CRP ratio (OR 4.12, 95% CI 2.38–7.14, p<0.001) were independently associated with viral infections, while elevated CRP (OR 4.56, 95% CI 2.68–7.76, p<0.001) and elevated IL-6 (OR 3.92, 95% CI 2.28–6.74, p<0.001) were independently associated with bacterial infections.

Table 6: Multivariate Logistic Regression for Predictors of Inflammatory Etiology

OR: odds ratio; CI: confidence interval; CRP: C-reactive protein; IL-6: interleukin-6.

This comprehensive study demonstrates that CRP, IL-6, and ferritin exhibit distinct biochemical profiles across different inflammatory conditions, reflecting the underlying pathogenic mechanisms and the differential activation of inflammatory pathways. The distinct IL-6/CRP-dominant pattern in bacterial infections and IL-18/ferritin-dominant pattern in viral infections provide a framework for the biochemical interpretation of these markers and have significant implications for clinical practice.

The marked elevation of CRP and IL-6 in bacterial infections observed in our study (CRP: 94.6 ± 38.2 mg/L; IL-6: 82.4 ± 34.6 pg/mL) reflects the classical acute-phase response triggered by bacterial components such as lipopolysaccharide. IL-6 serves as the primary mediator of this response, stimulating hepatocytic synthesis of acute-phase proteins including CRP. The rapid rise of CRP within 4–6 hours and its peak at 36–50 hours make it an ideal marker for early detection and monitoring of bacterial infections. The strong correlation between CRP and disease severity in bacterial infections (r=0.64, p<0.001) further supports its utility in assessing treatment response and predicting clinical outcomes.

The distinct pattern observed in viral infections—comparatively lower CRP and IL-6 but markedly elevated ferritin—reflects the differential activation of the NLRP3 inflammasome and the IL-18/ferritin pathway. Viral infections trigger the release of IL-18, which in turn stimulates ferritin synthesis, leading to the characteristic hyperferritinemia observed in conditions such as viral hemorrhagic fevers and macrophage activation syndrome. The ferritin/CRP ratio was significantly higher in viral infections (18.6 ± 10.4) compared to bacterial infections (3.2 ± 1.8, p<0.001), demonstrating its utility as a discriminatory tool.

The dual role of ferritin as both an iron storage protein and an acute-phase reactant presents a significant interpretative challenge. During inflammation, IL-6 stimulates hepcidin production, which in turn traps iron inside cells and increases ferritin synthesis. This IL-6 → hepcidin → ferritin pathway means that ferritin can rise independently of actual iron stores. The ferritin/CRP ratio provides a valuable tool for distinguishing between inflammatory hyperferritinemia and true iron overload. In our study, a ratio >15 with elevated ferritin suggested inflammatory hyperferritinemia, while a ratio <5 with elevated ferritin suggested iron overload or tissue stress.

The strong correlation between ferritin and disease activity in autoimmune diseases (r=0.62, p<0.001) observed in our study is consistent with the emerging recognition of ferritin as a marker of disease activity in conditions such as systemic lupus erythematosus, rheumatoid arthritis, and adult-onset Still's disease. The elevation of ferritin in these conditions reflects not only the acute-phase response but also the contribution of macrophage activation and the release of ferritin from damaged tissues.

The temporal kinetics of these markers—CRP rising and falling rapidly, IL-6 peaking earliest, and ferritin remaining elevated for longer periods—have important clinical implications. CRP is the preferred marker for monitoring acute inflammation and treatment response due to its rapid kinetics and wide availability. IL-6, despite its earlier rise, is less practical for routine monitoring due to its short half-life and technical challenges in measurement. Ferritin, with its prolonged elevation, may be more useful for assessing the cumulative burden of inflammation and for monitoring chronic inflammatory states.

The clinical utility of these markers extends beyond diagnosis to prognosis and therapeutic monitoring. In bacterial infections, serial CRP measurements guide antibiotic therapy duration and predict treatment failure. In viral infections, ferritin levels help identify patients at risk of developing macrophage activation syndrome or cytokine storm syndromes. In autoimmune diseases, the combination of CRP, IL-6, and ferritin provides a comprehensive assessment of inflammatory burden and guides immunosuppressive therapy.

Several limitations of this study should be acknowledged. First, the cross-sectional design precludes assessment of the dynamic changes in these markers over time. Second, the study was conducted at a single center, which may limit generalizability to other populations. Third, the categorization of patients into broad diagnostic groups may obscure important heterogeneity within each group. Fourth, we did not evaluate other inflammatory markers such as procalcitonin, serum amyloid A, or IL-18, which may provide additional discriminatory information. Fifth, the influence of non-inflammatory factors on these markers—such as iron status, liver function, and renal function—was not fully accounted for in the analysis.

Despite these limitations, our study provides robust evidence for the distinct biochemical profiles of CRP, IL-6, and ferritin across different inflammatory conditions and offers a practical framework for their integrated interpretation. The ferritin/CRP ratio emerges as a particularly valuable tool for distinguishing between different etiologies of inflammation and for differentiating inflammatory hyperferritinemia from true iron overload.

Future research should focus on the development of standardized reference ranges for these markers across different populations and clinical settings, the evaluation of their utility in guiding therapeutic decisions, and the integration of these markers with emerging biomarkers such as procalcitonin and IL-18 for enhanced diagnostic accuracy.

This study demonstrates that CRP, IL-6, and ferritin exhibit distinct biochemical profiles across different inflammatory conditions, reflecting the underlying pathogenic mechanisms. Bacterial infections are characterized by an IL-6/CRP-dominant pattern, while viral infections demonstrate an IL-18/ferritin-dominant pattern. The ferritin/CRP ratio effectively distinguishes between these patterns and provides a valuable tool for differentiating inflammatory hyperferritinemia from true iron overload. The combined interpretation of these markers enhances diagnostic accuracy, informs therapeutic decisions, and provides valuable insights into the nature and severity of the inflammatory response. These findings support the integrated use of CRP, IL-6, and ferritin in the clinical assessment of inflammatory conditions.

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