Understanding Iron Status: Serum Ferritin is More Reliable than Serum Iron

Article written by Steve Stone, PA-C, at Bronson Oncology and Hematology Specialists.

Iron is an essential mineral that plays a crucial role in various bodily functions, including the production of red blood cells. Proper assessment of iron status is important in diagnosing and managing iron deficiency. While serum iron levels are commonly tested, it is not a reliable indicator of iron status. Serum ferritin is a much better measure, although it has its challenges too. Here’s why serum ferritin is a better indicator and what you should be aware of when interpreting these levels.

Why Serum Iron is Not Enough

• Fluctuations: Serum iron levels can fluctuate throughout the day and are influenced by diet. This makes serum iron testing less reliable for diagnosing iron deficiency.
• Not Specific: Low serum iron can occur in other conditions besides iron deficiency, such as inflammation. It doesn't give a clear picture of your body’s iron stores.

The Importance of Serum Ferritin

• Storage Indicator: Ferritin is a protein that stores iron in your body. Serum ferritin levels directly reflect the amount of iron stored in your body. One can think of it as a measure of how much iron is “in the bank.”  The serum iron is highly variable, much like the amount of cash you may have “in your pocket” at any given moment.
• Reliable: Ferritin is less influenced by daily variations compared to serum iron. This makes it a more stable and reliable measure of iron status. It is important to realize it will rise with inflammation, potentially masking iron deficiency.

Problems with Ferritin Reference Ranges

• Wide Range: The reference range for ferritin in many labs is very broad. For example, a typical range might be 15-150 ng/mL. This wide range can lead to underdiagnosing iron deficiency.
• Misleading "Normal" Levels: Even if the ferritin is within the normal range, it doesn't mean iron stores are sufficient. A ferritin level under 50 ng/mL, though considered "normal," might indicate low iron stores.
• Actionable Thresholds:
  • Ferritin < 30 ng/mL: Even though this level might be considered normal by some labs, it’s a strong indicator of iron deficiency and needs to be addressed.
  • Ferritin 30-50 ng/mL: Levels in this range should be monitored closely as they might indicate borderline iron stores, especially if there are symptoms of iron deficiency.

What is Pagophagia and Why It Matters

• Pagophagia: This is the craving and chewing of ice. This is a specific sign of iron deficiency, even when ferritin levels are in the low-normal range.
• Red Flag: If someone experiences pagophagia, it is important to check their ferritin levels.

What Should Be Done?

• Re-evaluate "Normal" Ranges: The medical community needs to reconsider what’s truly “normal” when it comes to ferritin levels. A cutoff of 30 ng/mL is advisable clinically, but 50 ng/mL might be more appropriate for identifying iron deficiency.
• Monitor Ferritin Closely: Especially in women, ferritin levels should be monitored closely. Levels under 50 ng/mL should be taken seriously and anything under 30 ng/mL should prompt action.
• Educate: Healthcare providers and patients should be educated about the limitations of serum iron and the importance of ferritin. Awareness is key to improving diagnosis and treatment.  Avoid using the serum iron to guide decision making.

Key Takeaways

• Don’t Rely on Serum Iron: It’s not a reliable test for iron deficiency.
• Trust Ferritin but Understand its Limits: Ferritin is the best measure we have, but it’s important to interpret it correctly, especially in the context of inflammation or when approaching the low end of normal.
• Act on Low Ferritin: Don’t ignore ferritin levels under 50 ng/mL and be especially cautious with levels under 30 ng/mL.
• Watch for Specific Symptoms: Symptoms like pagophagia can be a strong indicator of iron deficiency, even with a low-normal ferritin level.

In summary, while serum ferritin is the most reliable test for assessing iron status, its reference ranges need to be interpreted carefully. Understanding the limitations of these ranges. Do not rely on serum iron levels when assessing iron status.

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