Medical Workup for Serum Iron 18 µg/dL and Transferrin Saturation 5%
These values indicate severe iron deficiency requiring immediate confirmation with serum ferritin, complete blood count, and comprehensive gastrointestinal evaluation to identify the underlying cause, as this degree of iron depletion strongly suggests pathologic blood loss or malabsorption. 1
Immediate Laboratory Assessment
Obtain serum ferritin as the single most useful confirmatory test for iron deficiency, with values <45 µg/L confirming absolute iron deficiency in the context of your patient's transferrin saturation of 5%. 1
Complete blood count with indices to assess for anemia (hemoglobin <13 g/dL in men, <12 g/dL in women) and determine if microcytic anemia is present. 1, 2
Screen for celiac disease with tissue transglutaminase antibodies (IgA-TTG), as celiac disease is found in 3-5% of iron deficiency anemia cases and represents a treatable cause of malabsorption. 1
Urinalysis or urine microscopy to exclude occult hematuria as a source of blood loss. 1
Critical Context: Transferrin Saturation <5% is Highly Abnormal
Your patient's transferrin saturation of 5% is severely reduced and indicates profound iron deficiency. While transferrin saturation <20% is the standard threshold for iron deficiency, values this low (5%) are particularly concerning. 1 In adults without inflammatory conditions, this degree of reduction strongly suggests either significant ongoing blood loss or severe malabsorption. 1, 2
Gastrointestinal Evaluation
For men and postmenopausal women with confirmed iron deficiency:
Bidirectional endoscopy (gastroscopy and colonoscopy) should be performed urgently as first-line investigation to exclude gastrointestinal malignancy and other bleeding sources. 1, 2
If colonoscopy is not feasible, CT colonography is a reasonable alternative for evaluating the colon. 1
Evaluate for Helicobacter pylori infection during gastroscopy, as this is a recognized cause of iron malabsorption. 1
For premenopausal women:
Detailed menstrual history is essential, as heavy menstrual bleeding is a common cause of iron deficiency in this population. 1
Gastrointestinal evaluation should still be considered if menstrual losses do not adequately explain the severity of iron deficiency or if there are any gastrointestinal symptoms. 1
Additional Considerations Based on Clinical Context
If ferritin is between 45-100 µg/L (equivocal range):
- Consider additional iron studies including soluble transferrin receptor or reticulocyte hemoglobin equivalent to confirm true iron deficiency, particularly if inflammatory conditions are present. 1, 3
If inflammatory conditions are suspected (CKD, heart failure, IBD):
Ferritin up to 100 µg/L may still reflect iron deficiency in the presence of inflammation, as ferritin is an acute-phase reactant. 1
Chronic kidney disease: Absolute iron deficiency is defined as transferrin saturation ≤20% with ferritin ≤100 µg/L (predialysis/peritoneal dialysis) or ≤200 µg/L (hemodialysis). 1
Heart failure: Iron deficiency is defined as ferritin <100 µg/L and/or transferrin saturation <20%, with low transferrin saturation independently associated with worse outcomes. 1, 4, 5
Dietary and Absorption Assessment
Evaluate dietary iron intake including consumption of heme iron sources (meat, poultry, fish) versus non-heme iron sources (plant-based). 1
Review medications that may impair iron absorption, including proton pump inhibitors, H2-receptor antagonists, and antacids. 1
Assess for malabsorptive conditions beyond celiac disease, including inflammatory bowel disease, previous gastric surgery, or atrophic gastritis. 1
If Initial Workup is Negative
For patients with negative bidirectional endoscopy but persistent or recurrent iron deficiency:
Capsule endoscopy is the preferred test for small bowel evaluation to identify occult bleeding sources such as angioectasias or small bowel lesions. 1
Repeat evaluation of the renal tract if not already performed. 1
Common Pitfalls to Avoid
Do not delay gastrointestinal evaluation in men or postmenopausal women, as 9% of patients over 65 years with iron deficiency anemia have gastrointestinal cancer. 2
Do not assume menstrual losses alone explain severe iron deficiency in premenopausal women without thorough evaluation, particularly if transferrin saturation is this profoundly reduced. 1
Do not rely on transferrin saturation alone in children, as normal developmental changes result in lower values, and 19.9% of iron-replete children have saturation <15%. 6, 7
Remember that serum iron and transferrin saturation exhibit circadian variation, so timing of blood draws should be consistent, preferably in the morning. 8