Interpretation of a Ferritin Level of 21
A ferritin level of 21 μg/L indicates early iron store depletion and is consistent with iron deficiency, particularly in women and children. 1
Understanding Ferritin and Iron Stores
Ferritin is the most specific indicator available for assessing iron stores in the body. Under normal conditions:
- 1 μg/L of serum ferritin corresponds to approximately 10 mg of stored iron 1
- Average ferritin levels in the US population are:
- Men: 135 μg/L
- Women: 43 μg/L
- Children (6-24 months): approximately 30 μg/L 1
Clinical Significance of a Ferritin Level of 21
Interpretation Based on Reference Values
- A ferritin level of 21 μg/L falls below normal ranges for men and is at the lower end of normal for women 1
- This level represents early depletion of iron stores before anemia develops 1
- According to CDC guidelines, ferritin is an early indicator of iron status and the most specific indicator of depleted iron stores 1
Diagnostic Thresholds
- Traditional cutoff for definite iron deficiency: ≤15 μg/L (sensitivity 75%, specificity 98%) 1
- In patients with inflammation, infection, or malignancy, iron deficiency may be present with ferritin levels up to 50 μg/L or even 100 μg/L 1, 2
- A ferritin level of 21 μg/L in a patient without inflammation strongly suggests iron deficiency 1, 2
Clinical Implications
Need for Further Assessment
- Additional iron status tests should be considered:
Risk of Progression
- A ferritin level of 21 μg/L indicates depleted iron stores that may progress to iron deficiency anemia if not addressed 3
- Functional studies show that iron deficiency anemia begins to develop when ferritin levels fall below 10 μg/L 4
Management Considerations
- Iron supplementation should be considered to replenish iron stores 1, 5
- Oral iron (such as ferrous sulfate 200 mg three times daily) is typically first-line therapy 1
- Treatment should continue for three months after correction of anemia to fully replenish iron stores 1
- Investigation for causes of iron deficiency may be warranted, especially in men and postmenopausal women 1, 6
Common Causes of Low Ferritin
- Blood loss (menstrual, gastrointestinal) 3
- Impaired iron absorption (atrophic gastritis, celiac disease) 3
- Inadequate dietary iron intake 3
- Pregnancy (increased iron requirements) 3
- Regular blood donation 5
Important Caveats
- Ferritin is an acute-phase reactant; inflammation, infection, or tissue damage can raise ferritin levels independent of iron status 1
- A "normal" ferritin level in the presence of inflammation may mask iron deficiency 1, 2
- In elderly patients, even a ferritin level ≤50 μg/L justifies gastrointestinal investigation to rule out pathology 6
- Ferritin levels should be interpreted in conjunction with other iron parameters and clinical context 1