Elevated Hemoglobin: Terminology and Clinical Context
The term for elevated hemoglobin is "erythrocytosis" or "polycythemia," with erythrocytosis referring specifically to increased red blood cell mass and polycythemia encompassing both true increases in red cell mass and relative increases due to plasma volume depletion. 1
Defining Elevated Hemoglobin Thresholds
- Erythrocytosis is defined as hemoglobin >18.5 g/dL in men or >16.5 g/dL in women, or hematocrit >55% in men or >49.5% in women. 1
- Hemoglobin levels >18.0 g/dL and hematocrit >52% in males are considered elevated and potentially indicative of erythrocytosis. 1
- These thresholds are critical because elevated levels are associated with increased thrombotic risk, particularly when hemoglobin exceeds 18.5 g/dL. 1
Primary vs. Secondary Terminology Distinctions
Primary polycythemia refers to polycythemia vera, a myeloproliferative neoplasm characterized by JAK2 mutations (present in up to 97% of cases) causing autonomous red blood cell production. 1, 2
Secondary erythrocytosis results from elevated erythropoietin production, either appropriate (hypoxia-driven) or inappropriate (tumor-related), and includes:
- Hypoxia-driven causes: chronic obstructive pulmonary disease, obstructive sleep apnea, cyanotic congenital heart disease, high altitude, and smoking ("smoker's polycythemia" from carbon monoxide exposure). 1
- Hypoxia-independent causes: renal cell carcinoma, hepatocellular carcinoma, pheochromocytoma, uterine leiomyoma, meningioma, and exogenous erythropoietin therapy. 1
Relative polycythemia (also called "Gaisböck syndrome" or "stress polycythemia") occurs when plasma volume depletion causes elevated hemoglobin/hematocrit without true increase in red cell mass, seen with dehydration, diuretic use, or burns. 1
Clinical Significance and Common Pitfalls
- Hemoglobin is the preferred measurement over hematocrit because it remains stable during sample storage, while hematocrit can falsely increase by 2-4% with prolonged storage and is affected by hyperglycemia. 1
- The term "polycythemia" is often used interchangeably with "erythrocytosis" in clinical practice, though technically polycythemia can refer to increases in all blood cell lines. 2
- Iron deficiency can coexist with erythrocytosis, causing microcytic polycythemia with elevated RBC count but paradoxically reduced hemoglobin—a critical diagnostic consideration that requires serum ferritin and transferrin saturation testing. 1
- Physiological variations exist: males have higher baseline hemoglobin than females due to testosterone effects, and altitude adaptation causes normal increases (e.g., +1.9 g/dL at 3,000 meters elevation). 1