Is a red blood cell count of 5.89 × 10⁶/µL within the normal range for an asymptomatic 18‑year‑old male?

RBC Count of 5.89 × 10⁶/µL in an 18-Year-Old Male

An RBC count of 5.89 × 10⁶/µL in an asymptomatic 18-year-old male is at the upper end of the normal range and does not require intervention, though it warrants verification of hemoglobin and hematocrit values to exclude polycythemia.

Normal Reference Ranges and Clinical Context

• The typical reference range for adult males is approximately 4.5–5.9 × 10⁶/µL, placing this value at the upper limit of normal 1.
• In the context of polycythemia evaluation, true polycythemia is defined by elevated red blood cell mass, not just RBC count alone—hemoglobin and hematocrit values are essential to distinguish true from apparent polycythemia 1.
• Hemoglobin levels above the 95th percentile (which varies by sex and race) may represent extreme normal values rather than pathology, and not all values outside the 2 SD range indicate disease 1.

Differential Considerations

Physiologic Variations

• Young adult males, particularly those who are physically active or live at higher altitudes, may have RBC counts at the upper end of normal due to physiologic adaptation 2.
• Smoking causes secondary polycythemia through chronic carbon monoxide exposure, which resolves with smoking cessation 1.

When to Investigate Further

• If hemoglobin is elevated (>16.5 g/dL in males) or hematocrit is elevated (>49% in males), further evaluation for polycythemia vera or secondary polycythemia is warranted 1.
• Polycythemia vera typically presents with elevated RBC mass plus JAK2 V617F mutation in approximately 95% of cases, along with potential splenomegaly and thrombotic complications 1.
• Secondary polycythemia may be hypoxia-driven (chronic lung disease, sleep apnea, high altitude) or hypoxia-independent (renal tumors, hepatocellular carcinoma, inappropriate EPO production) 1.

Recommended Approach

Immediate Assessment

• Verify the complete blood count including hemoglobin, hematocrit, and mean corpuscular volume (MCV) to assess whether this represents isolated RBC elevation or true erythrocytosis 1.
• Review the peripheral blood smear if hemoglobin/hematocrit are also elevated to assess red cell morphology 1.

Clinical History Focus

• Assess for smoking history, chronic hypoxemia symptoms (dyspnea, cyanosis), sleep disturbances suggesting sleep apnea, recent altitude exposure, or family history of polycythemia 1.
• Evaluate for symptoms of hyperviscosity (headache, dizziness, visual disturbances, pruritus after bathing) or thrombotic events, which would suggest polycythemia vera 1.

Management Algorithm

• If hemoglobin and hematocrit are normal: No further workup needed; this represents a normal variant 1.
• If hemoglobin >16.5 g/dL or hematocrit >49%: Repeat CBC in 2–4 weeks to confirm persistence, then proceed with JAK2 V617F mutation testing and serum erythropoietin level 1.
• If JAK2 V617F is positive with elevated hemoglobin/hematocrit: Diagnose polycythemia vera and initiate phlebotomy to maintain hematocrit <45% in males 1.
• If JAK2 V617F is negative with low serum EPO: Consider bone marrow biopsy and additional molecular testing for polycythemia vera 1.
• If serum EPO is elevated or normal: Investigate secondary causes including pulse oximetry, arterial blood gas, chest imaging, renal ultrasound, and sleep study as clinically indicated 1.

Critical Pitfalls to Avoid

• Do not order red blood cell mass measurements in obviously dehydrated patients or those with clear reactive causes of hemoconcentration, as this represents relative polycythemia that resolves with rehydration 1.
• Do not assume an isolated RBC count at the upper limit of normal indicates disease—hemoglobin and hematocrit must also be elevated to warrant further investigation 1.
• Do not overlook smoking as a reversible cause of elevated RBC parameters 1.