How Anemia Affects Erythrocyte Sedimentation Rate (ESR)
Anemia significantly elevates ESR values, with lower hematocrit levels directly correlating to higher ESR readings regardless of underlying inflammation. 1
Mechanism of ESR Elevation in Anemia
Anemia affects ESR through several mechanisms:
- Reduced hematocrit effect: Lower red blood cell count decreases the ratio of cells to plasma, allowing faster sedimentation
- Rouleaux formation: With fewer RBCs, the relative concentration of plasma proteins increases, enhancing rouleaux formation and accelerating sedimentation
- Altered plasma viscosity: Changes in the ratio of cells to plasma proteins alter blood viscosity
Quantifying the Effect of Anemia on ESR
The relationship between anemia and ESR elevation is substantial:
A study comparing automated and manual ESR measurements in anemic patients found significant ESR elevations across all ranges, with mean differences between corrected and automated ESR values ranging from 2.33 mm/hr in low ESR ranges to 43.3 mm/hr in very high ESR ranges 1
Research examining extremely elevated ESR (≥100 mm/hr) demonstrated a significant negative correlation between ESR and red blood cell count (RBC), hemoglobin (Hb), and hematocrit (HCT) levels (correlation coefficients of -0.395, -0.381, and -0.383 respectively) 2
Clinical Implications and Correction Methods
To account for anemia's effect on ESR:
Hematocrit correction formula: The hematocrit-corrected ESR calculation [ESR × (Hct/45)] can provide more accurate assessment of inflammation in anemic patients 3
In hemodialysis patients, using the hematocrit-corrected ESR improved diagnostic accuracy for inflammation with 100% sensitivity and specificity 3
For patients with low hematocrit, automated ESR measurement methods may be reliable for low ESR values, but validation by the standard Westergren method is recommended for high ESR values 1
Clinical Pitfalls to Avoid
Misinterpreting elevated ESR: Don't assume inflammation is present based solely on elevated ESR in anemic patients without correcting for hematocrit
Overlooking multifactorial causes: In elderly patients with elevated ESR, multiple diagnoses often contribute simultaneously 4
Neglecting correction in specific populations: Particularly important in patients with chronic kidney disease or on hemodialysis, where anemia and inflammation often coexist 3
Assuming correlation with CRP: Unlike other inflammatory markers, extremely elevated ESR doesn't necessarily correlate with CRP levels (correlation coefficient -0.019) 2
When evaluating ESR results in anemic patients, always consider the hematocrit level and apply appropriate correction methods to avoid misdiagnosis and inappropriate treatment decisions.