Iron Deficiency Diagnosis and Treatment Recommendation
You have iron deficiency and should start oral iron supplementation with ferrous sulfate 325 mg daily (or on alternate days to improve tolerability), while investigating the underlying cause of your iron deficiency. 1
Interpretation of Your Lab Values
Your ferritin of 31 ng/mL and transferrin saturation of 19% both indicate absolute iron deficiency:
- Ferritin <45 ng/mL has 85% sensitivity and 92% specificity for iron deficiency 2
- Transferrin saturation <20% is a diagnostic criterion used across multiple guidelines 3, 1, 4
- Your values fall below both thresholds, confirming iron deficiency even without knowing your hemoglobin level
The recent 2025 JAMA review establishes that iron deficiency is diagnosed by ferritin typically <30 ng/mL in individuals without inflammatory conditions, or by transferrin saturation <20% 1. Your ferritin of 31 ng/mL sits right at this threshold, and your transferrin saturation of 19% confirms the diagnosis.
Treatment Approach
First-Line: Oral Iron Therapy
Start with ferrous sulfate 325 mg daily or on alternate days 1. The alternate-day dosing may improve tolerability while maintaining efficacy. This is appropriate for most patients without contraindications.
When to Consider Intravenous Iron
Switch to IV iron if you have: 1
- Oral iron intolerance (gastrointestinal side effects)
- Malabsorption conditions (celiac disease, post-bariatric surgery, inflammatory bowel disease)
- Chronic kidney disease
- Heart failure
- Active cancer
- Ongoing blood loss
- Pregnancy (second or third trimester)
Mandatory: Identify the Underlying Cause
You need evaluation to find why you're iron deficient. The most common causes are: 1
- Bleeding: Menstrual (in premenopausal women) or gastrointestinal
- Malabsorption: Atrophic gastritis, celiac disease, bariatric surgery
- Inadequate dietary intake: Vegetarian/vegan diet
- Medications: NSAIDs causing occult GI bleeding
Endoscopic Evaluation
If you are a postmenopausal woman or a man, the AGA strongly recommends bidirectional endoscopy (both upper endoscopy and colonoscopy) to evaluate for gastrointestinal sources of blood loss 2. This should be performed unless there is an obvious alternative explanation (frequent blood donation, documented dietary deficiency, known malabsorption).
For premenopausal women with heavy menstrual bleeding, gynecologic evaluation may be sufficient without immediate endoscopy.
Treatment Targets
Aim for:
- Hemoglobin normalization (if anemic)
- Ferritin >100 μg/L in most conditions 4
- Transferrin saturation >20% 4
Important Caveats
Transferrin saturation is emerging as the more clinically meaningful marker compared to ferritin, particularly for predicting outcomes. Recent 2025 research in heart failure patients shows that low transferrin saturation (not ferritin) independently predicts mortality and adverse outcomes 5, 6. While you don't have documented heart failure, this highlights that your transferrin saturation of 19% is the more concerning value and warrants treatment.
Ferritin can be falsely elevated by inflammation, chronic kidney disease, liver disease, or malignancy 2, 3. In these conditions, ferritin up to 100 ng/mL may still represent iron deficiency, making transferrin saturation more reliable 3.