Is low dietary iron the cause of iron deficiency in a patient with low iron levels and a normal Complete Blood Count (CBC)?

Low Iron with Normal CBC: Is Dietary Insufficiency the Cause?

No, low dietary iron should not be presumed as the cause of iron deficiency even when the CBC is normal, and a full gastrointestinal investigation is still required in most cases. 1

Key Principle: Non-Anemic Iron Deficiency Still Requires Investigation

Iron deficiency progresses through stages, beginning with depleted iron stores (low ferritin) before anemia develops. 1 This "non-anemic iron deficiency" (NAID) is common—for example, over half of young women with menorrhagia have reduced iron stores but only 25% are actually anemic. 1

Critical caveat: While dietary insufficiency is common, the British Society of Gastroenterology explicitly states that "a positive dietary history should not be presumed as the cause of anaemia and a full GI investigation is still required." 1 This is because serious underlying pathology, including malignancy, can present with iron deficiency before anemia develops.

When Dietary Iron Deficiency is the Likely Cause

Low dietary iron is most probable in these specific populations:

• Premenopausal women with menorrhagia 1
• Pregnant women (up to 84% have iron deficiency in third trimester) 2
• Vegetarians/vegans (non-heme iron has substantially lower bioavailability) 1
• Young women with heavy menstrual bleeding 2

In premenopausal women with NAID and no other concerning features, GI investigation is generally not warranted. 1 However, this is the exception, not the rule.

Who Requires Full Investigation Despite Normal CBC

All of the following require gastrointestinal evaluation even without anemia: 1

• Men of any age with iron deficiency
• Postmenopausal women with iron deficiency
• Anyone with ferritin <30 μg/L and risk factors for GI pathology
• Patients using NSAIDs or anticoagulants
• Those with GI symptoms (even if mild)
• Patients with family history of GI malignancy

The overall prevalence of significant underlying GI pathology in NAID is low, but the consequences of missing malignancy are severe. 1

Diagnostic Approach

Confirm iron deficiency first:

• Serum ferritin <15 μg/L is diagnostic of iron deficiency (specificity 0.99) 1
• Ferritin <30 μg/L generally indicates low body iron stores 1
• Transferrin saturation <20% supports the diagnosis 2

Then determine the cause:

1. Take dietary history to identify iron-deficient diets, but do not stop there 1

2. Assess for blood loss:

   • Menstrual history in premenopausal women
   • NSAID use (stop if possible) 1
   • Stool guaiac for occult GI bleeding 1

3. Screen for malabsorption:

   • Celiac serology (antiendomysial antibody) 1
   • History of bariatric surgery 2
   • History of atrophic gastritis 2

4. Proceed to endoscopy if indicated (see above criteria) 1

Treatment Trial as Diagnostic Tool

If investigation is deferred (only appropriate in low-risk premenopausal women), a therapeutic trial of oral iron can confirm dietary deficiency:

• Oral ferrous sulfate 325 mg daily or on alternate days 2
• Response within 3 weeks confirms true iron deficiency 1
• Lack of response mandates full GI investigation 1

Common Pitfall to Avoid

The most dangerous error is attributing iron deficiency to diet in men or postmenopausal women without investigation. These patients have gastrointestinal pathology as the most common cause, and 30-50% will have findings on upper endoscopy alone. 1 Colorectal cancer can present with iron deficiency before anemia develops, and delaying investigation risks missing curable disease.

Bottom line: Unless the patient is a premenopausal woman with clear menstrual blood loss or dietary insufficiency, assume GI pathology until proven otherwise, regardless of whether the CBC shows anemia. 1