What laboratory tests should I undergo to rapidly assess the need for iron supplements?

Rapid Laboratory Assessment for Iron Supplementation Need

Order a serum ferritin and transferrin saturation (TSAT) as your initial tests—these two markers provide the fastest and most reliable assessment of whether you need iron supplements. 1, 2

First-Line Tests (Order These)

Serum Ferritin:

• Most sensitive and specific test for iron stores 1, 3
• Iron deficiency confirmed when <30 ng/mL in healthy individuals 3
• In the absence of inflammation, ferritin <50 ng/mL indicates iron deficiency regardless of sex 4
• For women: <11-15 ng/mL confirms insufficient iron stores 4, 1
• For men: <25 ng/mL indicates insufficient iron stores 1, 2

Transferrin Saturation (TSAT):

• Calculated as: (serum iron ÷ total iron binding capacity) × 100 4
• TSAT <20% has high sensitivity for diagnosing iron deficiency 4, 1, 2
• Less affected by inflammation than ferritin, making it a reliable complementary test 2
• Reflects iron immediately available for hemoglobin synthesis 4, 2

Complete Blood Count (CBC)

While CBC alone is insufficient for diagnosis, it provides supportive information:

• Hemoglobin and hematocrit are late indicators of iron deficiency—they only drop when deficiency is severe 1, 5
• Mean corpuscular volume (MCV) <75 fL has 83% predictive value for iron deficiency 6
• Red cell distribution width (RDW) >14.0% with microcytosis strongly suggests iron deficiency 1

Critical pitfall: Normal hemoglobin does NOT rule out iron deficiency—you can have depleted iron stores with normal CBC parameters 5

Special Circumstances Requiring Different Thresholds

If you have chronic inflammation, kidney disease, heart failure, or inflammatory bowel disease:

• Use ferritin <100 ng/mL as the cutoff (not <30 ng/mL) because inflammation falsely elevates ferritin 4, 2
• Rely more heavily on TSAT <20% in these conditions 4, 2
• Consider hepcidin testing if available, as it's more reliable than ferritin when inflammation is present 4

If you have chronic kidney disease specifically:

• Ferritin <100 ng/mL suggests iron deficiency 4, 2
• TSAT <20% indicates need for supplementation 4

Timing of Testing

• Do NOT check iron parameters within 4 weeks of receiving IV iron—circulating iron interferes with assays and gives falsely elevated results 4
• Recheck 4-8 weeks after starting supplementation to assess response 4
• Hemoglobin should increase by 1-2 g/dL within 4-8 weeks if treatment is working 4, 7

What NOT to Order Initially

• Serum iron alone is unreliable—it has high day-to-day variation and poor predictive value 4, 6
• Total iron binding capacity (TIBC) alone has insufficient predictive value for routine use 6
• Bone marrow biopsy is only needed if ferritin is equivocal (46-99 ng/mL) and diagnosis remains unclear 7
• Soluble transferrin receptor (sTfR) and reticulocyte hemoglobin content are specialized tests with limited availability—reserve for complex cases 4

Common Diagnostic Pitfalls to Avoid

• Relying only on CBC: Many iron-deficient patients have normal hemoglobin, hematocrit, and RBC indices until deficiency becomes severe 1, 5
• Ignoring inflammation: Ferritin is an acute-phase reactant—infection, chronic disease, liver disease, and malignancy can falsely elevate it, masking true iron deficiency 4, 2
• Ordering too many tests: Ferritin + TSAT are sufficient for most patients; additional tests add cost without improving diagnostic accuracy 1, 3
• Testing too soon after IV iron: Wait at least 4 weeks to avoid spurious results 4

Practical Algorithm

1. Order: Serum ferritin + TSAT 1, 2, 3
2. If ferritin <30 ng/mL OR TSAT <20%: Iron deficiency confirmed—start supplementation 4, 3
3. If you have chronic inflammatory conditions: Use ferritin <100 ng/mL as cutoff 4, 2
4. If results are discordant (e.g., high ferritin but low TSAT): Suspect functional iron deficiency from inflammation—TSAT <20% indicates need for iron despite elevated ferritin 4