Which laboratory test is most appropriate for screening iron deficiency?

Serum Ferritin is the Best Screening Test for Iron Deficiency

Serum ferritin is the most specific and recommended laboratory test for screening iron deficiency in clinical practice. Both the 2021 British Society of Gastroenterology and 2020 AGA guidelines consistently identify ferritin as the primary screening test 1, 2.

Why Ferritin is Superior

Serum ferritin directly reflects body iron stores and provides the highest specificity for iron deficiency in the absence of inflammation 1:

• Ferritin <15 μg/L: Highly specific for iron deficiency (specificity 99%) and indicates absent iron stores 1
• Ferritin <30 μg/L: Generally indicates low body iron stores 1
• Ferritin <45 μg/L: Optimal threshold balancing sensitivity (85%) and specificity (92%) for clinical screening 2, 1

The 45 μg/L threshold is recommended for screening because it maximizes detection of iron deficiency while maintaining acceptable specificity, reducing missed diagnoses that could delay identification of serious underlying pathology 2, 1.

Critical Caveat: The Inflammation Problem

Ferritin is an acute phase reactant and can be falsely elevated in inflammatory conditions, chronic disease, or infection 1. This is the single most important pitfall in ferritin interpretation:

• Even with true iron deficiency, ferritin may appear "normal" if inflammation is present
• Ferritin >150 μg/L makes absolute iron deficiency unlikely, even with inflammation 1
• When inflammation is suspected, consider adjunctive tests:
  • C-reactive protein (CRP) to identify inflammation
  • Transferrin saturation
  • Soluble transferrin receptor (sTfR)
  • The sTfR/log ferritin ratio (superior in chronic disease) 1

Supporting Tests (Not Primary Screening)

While other markers exist, they are less suitable for initial screening:

Red cell indices (MCV, MCH): Mean cell hemoglobin (MCH) may be more reliable than MCV and is less machine-dependent 1. However, both lose sensitivity in chronic disease, thalassemia, or B12/folate deficiency and lack specificity 1.

Reticulocyte hemoglobin content (Ret-Hb): Shows promise as an early marker of iron-deficient erythropoiesis and is less affected by inflammation than ferritin 3, 4, 5. However, it lacks standardized cutoff values and is not yet universally available or recommended in major guidelines 4.

Transferrin saturation: Useful adjunctively, particularly in inflammatory states, but not recommended as the sole screening test 2, 1.

Practical Screening Algorithm

1. Order serum ferritin as the primary screening test

2. Interpret based on clinical context:

   • <15 μg/L = definite iron deficiency
   • 15-45 μg/L = likely iron deficiency, proceed with evaluation
   • 45-150 μg/L = consider inflammation; check CRP and additional iron studies if clinical suspicion high

   • 150 μg/L = iron deficiency unlikely

3. If inflammation present (elevated CRP):

   • Add transferrin saturation
   • Consider sTfR or sTfR/log ferritin ratio if available
   • Use higher ferritin threshold (up to 100 μg/L may indicate deficiency with inflammation)

4. Always obtain complete blood count (CBC) to assess for anemia and red cell indices 1

The evidence consistently supports ferritin as the single best screening test, with the understanding that clinical judgment and additional testing are required when inflammation confounds interpretation.