What does ferritin measure?

What Ferritin Measures

Serum ferritin directly measures the amount of iron stored in the body, with 1 μg/L of serum ferritin concentration equivalent to approximately 10 mg of stored iron. 1

Relationship to Iron Stores

Ferritin is primarily an intracellular protein with a small amount circulating in the plasma. Under normal conditions, serum ferritin concentration has a direct relationship with the body's iron stores 2. This makes ferritin measurement particularly valuable for clinical assessment of iron status.

Key points about ferritin as an iron store indicator:

• It is the most specific indicator available for detecting depleted iron stores 2
• Low serum ferritin is an early indicator of reduced iron stores, appearing before other signs of iron deficiency 1
• Normal ranges vary by demographic:
  • Men: average 135 μg/L
  • Women: average 43 μg/L
  • Children (6-24 months): approximately 30 μg/L 2

Diagnostic Value

Ferritin is highly useful in diagnosing iron deficiency:

• Levels <15 μg/L have 98% specificity for iron deficiency 1
• When set at <12 μg/L, specificity increases to 100% 2
• Normal ferritin essentially rules out iron overload in hemochromatosis 2

Important Clinical Considerations

Inflammatory Confounding

The most significant limitation of ferritin measurement is its behavior as an acute-phase reactant. Ferritin levels can be elevated independent of iron status in:

• Chronic infection or inflammation
• Liver disease (hepatitis, cirrhosis)
• Malignancies
• Arthritis and other inflammatory conditions 2, 1

This elevation can mask truly depleted iron stores, making interpretation challenging in inflammatory states 1.

Diagnostic Algorithm

For accurate assessment of iron status:

1. Measure serum ferritin as the primary indicator
2. Consider inflammatory markers (CRP, WBC count) when interpreting results
3. In inflammatory conditions, use higher ferritin cutoffs (up to 100 μg/L) to diagnose iron deficiency 1
4. Combine ferritin with other iron parameters for comprehensive assessment:
   • Transferrin saturation
   • Serum iron
   • Total iron binding capacity
   • Complete blood count with red cell indices 1

Special Populations

• Pregnancy: Normal ferritin in early pregnancy may not predict adequate iron status later in pregnancy 2
• Chronic kidney disease: Ferritin may be elevated despite iron deficiency 1
• Inflammatory bowel disease: Higher cutoff values are recommended 1

Monitoring Therapy

Ferritin is valuable for monitoring iron therapy:

• Standard treatment doses of iron in adults typically don't cause ferritin rise until hemoglobin normalizes 3
• In iron overload conditions, ferritin monitoring helps guide chelation therapy 4
• Ferritin levels <500 μg/L indicate potential need to interrupt chelation therapy 4

Conclusion

Serum ferritin is the most reliable non-invasive method to assess iron stores, providing crucial information for diagnosing both iron deficiency and overload. However, its interpretation requires consideration of inflammatory status and should be part of a comprehensive iron assessment approach.