For a patient with normal vitamin B12 and hemoglobin, iron deficiency, low vitamin D, elevated C‑reactive protein, and mild leukocytosis as the only abnormal CBC finding, what is the appropriate next step in evaluation and management?

Workup for Mild Leukocytosis with Iron Deficiency

In a patient with iron deficiency, low vitamin D, elevated CRP, and mild leukocytosis as the only CBC abnormality, the priority is urgent gastrointestinal evaluation to exclude malignancy, followed by iron replacement therapy. The leukocytosis is likely reactive to the inflammatory state (elevated CRP) and does not require specific hematologic workup at this stage.

Immediate Next Steps

1. Gastrointestinal Investigation (Urgent)

You must pursue bidirectional endoscopy (gastroscopy and colonoscopy) urgently to exclude gastrointestinal malignancy and other pathology causing iron deficiency 1. This takes precedence even though the hemoglobin is currently normal, because:

• Iron deficiency without anemia still warrants the same diagnostic approach as iron deficiency anemia 1
• The elevated CRP suggests an inflammatory or pathologic process
• Delaying investigation risks missing early-stage malignancy

Before endoscopy, obtain:

• Celiac disease screening (tissue transglutaminase antibody) - found in 3-5% of iron deficiency cases 1
• Urinalysis to exclude renal blood loss 1
• Detailed dietary history and menstrual history if premenopausal woman

2. Iron Replacement - Start Immediately

Do not defer iron replacement while awaiting investigations 1. Begin oral iron supplementation now:

• Ferrous sulfate, fumarate, or gluconate: one tablet daily 1
• If not tolerated, reduce to every other day dosing 2, 1
• Take on empty stomach when possible, or with vitamin C 500mg to enhance absorption 2
• Recheck hemoglobin in 4 weeks - expect ≥10 g/L rise if truly iron deficient 1

3. Address the Elevated CRP Context

The elevated CRP is critical because inflammatory conditions can mask iron deficiency even with ferritin >45 mg/dL 2. In your patient:

• Ferritin may be falsely normal despite true iron deficiency
• Consider checking transferrin saturation or soluble transferrin receptor for confirmation 2, 1
• The mild leukocytosis (WBC elevation) is most likely reactive to the inflammatory state rather than a primary hematologic disorder

Common pitfall: Assuming normal ferritin excludes iron deficiency when CRP is elevated. In inflammatory states, ferritin <100 μg/L may still indicate iron deficiency 1.

Understanding the Leukocytosis

The mild WBC elevation in this context is almost certainly reactive and does not require immediate hematologic workup because:

• It occurs alongside elevated CRP (inflammatory marker)
• All other CBC parameters are normal
• There are no B symptoms or other concerning features mentioned
• Vitamin B12 is normal (excluding megaloblastic causes)

Monitor the WBC count - it should normalize as the underlying cause (likely GI pathology causing inflammation) is identified and treated.

Vitamin D Supplementation

While low vitamin D can correlate with hematologic parameters 3, this is not your priority. Address it after the urgent GI evaluation:

• Standard vitamin D replacement (cholecalciferol 1000-2000 IU daily or weekly high-dose)
• This will not resolve the iron deficiency or leukocytosis

If Initial Endoscopy is Negative

Should gastroscopy and colonoscopy be unrevealing:

1. Ensure adequate quality of the examinations 1
2. Assess response to iron therapy - if hemoglobin rises appropriately, continue iron and monitor 1
3. If inadequate response or recurrent iron deficiency: proceed to small bowel evaluation with capsule endoscopy (preferred) or CT/MR enterography 1

Algorithm Summary

Iron deficiency + elevated CRP + mild leukocytosis
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1. Order celiac serology + urinalysis
2. Start oral iron (ferrous salt, 1 tablet daily)
3. Arrange urgent bidirectional endoscopy
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Endoscopy results + recheck CBC/iron studies in 4 weeks
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If negative endoscopy but poor iron response → capsule endoscopy
If leukocytosis persists after treating underlying cause → hematology referral

The leukocytosis requires no specific intervention now - it will be reassessed after addressing the iron deficiency and its underlying cause. Only if it persists or worsens after successful treatment would you consider primary hematologic evaluation.