What are the possible causes of elevated hemoglobin, hematocrit, and red‑cell distribution width, and how should they be evaluated?

Elevated Hemoglobin, Hematocrit, and RDW: Evaluation Approach

When you encounter elevated hemoglobin, hematocrit, AND red cell distribution width together, you must first rule out polycythemia vera (PV) by testing for JAK2 mutation, as this combination can represent a myeloproliferative neoplasm that carries significant thrombotic risk and mortality if untreated.

Primary Differential Diagnosis

The combination of elevated Hgb/Hct with elevated RDW creates a specific diagnostic pattern that narrows your differential considerably:

1. Polycythemia Vera (Most Critical to Rule Out)

Test for JAK2 mutation immediately in any patient with:

• Hemoglobin >16.5 g/dL (men) or >16 g/dL (women)
• Hematocrit >49% (men) or >48% (women)
• Elevated RDW 1, 2

The diagnostic algorithm per WHO 2016 criteria requires 2:

Major Criteria:

1. Elevated Hgb/Hct above thresholds listed
2. Bone marrow showing hypercellularity with trilineage myeloproliferation
3. Presence of JAK2 mutation (present in >97% of PV cases)

Minor Criterion:

• Subnormal serum erythropoietin level

Diagnosis requires: All 3 major criteria OR first 2 major criteria + the minor criterion.

2. Iron Deficiency with Secondary Erythrocytosis

This is a critical pitfall: Iron deficiency paradoxically can present with elevated Hgb/Hct AND elevated RDW 1. The elevated RDW reflects anisocytosis from iron deficiency, while the elevated Hgb/Hct may represent:

• Chronic hypoxia-driven erythrocytosis (COPD, high altitude, sleep apnea)
• Hemoconcentration
• Congenital heart disease (particularly post-Fontan patients) 3

Key distinguishing features:

• Check iron studies: low ferritin, low transferrin saturation (<27%), elevated TIBC 3
• RDW typically >14% in iron deficiency 4, 5
• MCV may be normal initially, then becomes low as iron deficiency progresses 4

Important caveat: In PV patients with concurrent iron deficiency, the hemoglobin may be falsely lowered, masking the diagnosis. The WHO criteria acknowledge that formal PV diagnosis may require demonstrating elevated Hgb/Hct after iron replacement in research settings, though in clinical practice you should not delay diagnosis if other criteria are met 1.

3. Secondary Erythrocytosis

Elevated RDW with elevated Hgb/Hct can occur in:

• Chronic hypoxia: COPD, cyanotic heart disease, sleep apnea (check oxygen saturation, consider polysomnography) 6
• High altitude residence
• Renal cell carcinoma or other EPO-secreting tumors
• Post-renal transplant erythrocytosis

Distinguish from PV by:

• Normal or elevated serum erythropoietin (low in PV)
• Absence of JAK2 mutation
• Absence of splenomegaly
• Normal bone marrow histology

Diagnostic Algorithm

Step 1: Confirm true polycythemia (not hemoconcentration)

• Repeat CBC
• Check volume status clinically

Step 2: Order JAK2 V617F mutation testing immediately

• If positive → proceed with PV workup (bone marrow biopsy, EPO level)
• If negative → consider JAK2 exon 12 mutations in younger patients with isolated erythrocytosis 1

Step 3: Obtain iron studies simultaneously

• Ferritin, serum iron, TIBC, transferrin saturation
• RDW >14% with low ferritin suggests iron deficiency 4, 5

Step 4: Measure serum erythropoietin level

• Low EPO → supports PV diagnosis
• Normal/high EPO → suggests secondary erythrocytosis

Step 5: Evaluate for secondary causes

• Arterial oxygen saturation
• Chest imaging if hypoxia suspected
• Renal ultrasound if tumor suspected
• Sleep study if obstructive sleep apnea suspected 6

Step 6: If JAK2 negative and no secondary cause identified

• Consider CALR or MPL mutations (though these typically cause thrombocytosis, not isolated erythrocytosis)
• Bone marrow biopsy to evaluate for myeloproliferative features
• Check for high-oxygen-affinity hemoglobin variants or congenital causes

Clinical Significance of Elevated RDW

The elevated RDW in your patient is particularly important because:

• In PV: RDW elevation reflects heterogeneous red cell populations and active erythropoiesis 7
• In iron deficiency: RDW >14% has 77% sensitivity for iron deficiency anemia and can detect latent iron deficiency before anemia develops 4
• Prognostic marker: Elevated RDW independently predicts thrombotic risk, which is critical in both PV and secondary erythrocytosis 8

Critical Pitfalls to Avoid

1. Do not assume elevated Hgb/Hct with elevated RDW is simply iron deficiency - you must rule out PV first given its thrombotic and leukemic transformation risk
2. Do not wait for splenomegaly - only required in older WHO criteria; current criteria emphasize JAK2 mutation 1, 2
3. Do not dismiss the diagnosis if iron deficiency is present - PV patients frequently develop iron deficiency from phlebotomy or occult bleeding 1
4. Do not order red cell mass measurement routinely - it's been removed from essential criteria and is rarely needed with modern molecular testing 1