Which laboratory tests should I order for a woman with normal hemoglobin but iron deficiency to evaluate for occult gastrointestinal loss and related causes?

Laboratory Testing for Iron Deficiency Without Anemia

In a woman with normal hemoglobin but confirmed iron deficiency, order a complete blood count with red cell indices, serum ferritin, transferrin saturation, CRP, urinalysis, and tissue transglutaminase (TTG) antibodies to screen for celiac disease, followed by consideration of bidirectional endoscopy if occult gastrointestinal loss is suspected. 1

Essential Initial Laboratory Panel

Iron Studies and Inflammatory Markers

• Serum ferritin is the single most powerful test for iron deficiency, with levels <30 μg/L confirming iron deficiency in the absence of inflammation and <15 μg/L being diagnostic regardless of inflammatory status 1
• Transferrin saturation (TSAT) should be measured as it is less affected by inflammation than ferritin; TSAT <20% supports iron deficiency and helps identify functional iron deficiency 1
• C-reactive protein (CRP) is essential because ferritin is an acute-phase protein that can be falsely elevated in inflammatory states, potentially masking true iron deficiency 1
• In the presence of inflammation, ferritin up to 100 μg/L may still be consistent with iron deficiency 1

Complete Blood Count with Indices

• Mean corpuscular volume (MCV) and mean corpuscular hemoglobin (MCH) detect microcytosis and hypochromia, though MCH may be more sensitive for iron deficiency as it is less dependent on storage conditions 1
• Red cell distribution width (RDW) increases with iron deficiency and helps differentiate from other causes of microcytosis 1
• Reticulocyte count evaluates bone marrow response; low or normal reticulocytes indicate inability to respond properly to iron deficiency 1, 2

Screening for Occult Blood Loss

Urinalysis

• Urinalysis or urine microscopy should be performed to exclude urinary tract blood loss as a source of iron deficiency 1

Celiac Disease Screening

• Tissue transglutaminase (TTG) antibodies are mandatory, as celiac disease is found in 3-5% of cases of iron deficiency and can cause malabsorption leading to iron deficiency without overt anemia 1
• Patients must not have been excluding gluten for at least 6 weeks prior to testing, as this can cause false-negative results 1
• IgA levels should be checked if TTG is negative but suspicion remains high, as IgA deficiency is relatively common and causes falsely negative TTG readings 1

Fecal Occult Blood Testing

• Stool guaiac test for occult blood is recommended to test for gastrointestinal bleeding in patients with iron deficiency, though current evidence is insufficient to recommend faecal immunochemical testing for risk stratification 1

Additional Laboratory Tests to Consider

Vitamin Deficiencies

• Vitamin B12 and folate levels should be checked to exclude combined deficiency, particularly if macrocytosis is present or if there is extensive small bowel disease 1, 2
• These deficiencies are less prevalent in iron deficiency but should be considered in the comprehensive workup 1

Hemolysis Markers (If Indicated)

• Haptoglobin, lactate dehydrogenase, and bilirubin should be measured if reticulocyte count is elevated, suggesting hemolysis rather than simple iron deficiency 1

Critical Pitfalls to Avoid

• Do not rely on ferritin alone in patients with any clinical, endoscopic, or biochemical evidence of inflammation; use TSAT to confirm iron deficiency as ferritin can be falsely normal or elevated 1, 2
• Do not overlook celiac disease screening—it is a common and treatable cause of iron deficiency that may not present with gastrointestinal symptoms 1
• Do not assume dietary insufficiency alone without confirming with iron studies and excluding gastrointestinal pathology, especially in postmenopausal women where menstrual losses are not a factor 2, 3
• Non-anemic iron deficiency (NAID) generally does not warrant gastrointestinal investigation in premenopausal women unless other concerning features are present, as the prevalence of significant GI pathology is low in this population 1

When to Proceed to Endoscopic Evaluation

• If iron deficiency is confirmed and no obvious non-GI source is identified (menstrual losses, dietary insufficiency, malabsorption), bidirectional endoscopy (gastroscopy and colonoscopy) should be considered, particularly in postmenopausal women and men 1, 4
• Gastrointestinal lesions are frequently found in patients with iron deficiency, with upper GI lesions in approximately 13-36% and lower GI lesions (including malignancy) in 7-25% of cases 4, 5, 3
• The presence of gastrointestinal symptoms, positive fecal occult blood, or weight loss significantly increases the likelihood of finding pathology and should prompt endoscopic evaluation even in premenopausal women 3