Erythrocyte parameters

Erythrocytes (RBC) Overview

Erythrocytes, commonly known as Red Blood Cells (RBCs), are the most numerous type of blood cell. Their primary function is to transport oxygen from the lungs to the body's tissues and to facilitate the transport of carbon dioxide from the tissues back to the lungs. This vital function is carried out by hemoglobin, an iron-containing protein within the RBCs that binds to oxygen.

RBCs are produced in the bone marrow through a process called erythropoiesis and have a lifespan of approximately 120 days in circulation before being removed by the spleen and liver. They are unique among mammalian cells in that mature RBCs lack a nucleus and most organelles, maximizing space for hemoglobin. Their characteristic biconcave disc shape provides a large surface area for gas exchange and allows them to deform easily to pass through narrow capillaries.

RBCs in the Complete Blood Count (CBC)

Evaluation of red blood cells is a fundamental part of the Complete Blood Count (CBC), a common laboratory blood test. The CBC provides quantitative and qualitative information about RBCs, including:

• Red Blood Cell Count (RBC Count)
• Hemoglobin (HGB) concentration
• Hematocrit (HCT)
• Red Blood Cell Indices (MCV, MCH, MCHC, RDW)

These measurements help diagnose and classify various conditions, most notably anemia (low RBC count or hemoglobin) and polycythemia (high RBC count).

Red Blood Cell Count

This measures the number of red blood cells per unit volume of blood, typically expressed as cells x 10¹² per liter (x 10¹²/L) or cells x 10⁶ per microliter (x 10⁶/µL).

Erythrocytosis / Polycythemia (High RBC Count)

An increased number of red blood cells (generally >6.0 x 10¹²/L in men, >5.0-5.5 x 10¹²/L in women, though ranges vary) is termed erythrocytosis or polycythemia. It can be:

• Absolute Polycythemia: An actual increase in the total red cell mass due to increased production (erythropoiesis) in the bone marrow.
  • Primary Polycythemia: Uncontrolled RBC production independent of erythropoietin (EPO), most commonly Polycythemia Vera (a myeloproliferative neoplasm).
  • Secondary Polycythemia: Increased RBC production driven by elevated levels of erythropoietin (EPO). This can be:
    • Appropriate (Hypoxia-driven): Response to chronic low oxygen levels (e.g., high altitude, chronic lung disease, cyanotic congenital heart disease, smoking, sleep apnea like Pickwickian syndrome).
    • Inappropriate: EPO production by tumors (e.g., renal cell carcinoma, hepatocellular carcinoma, cerebellar hemangioblastoma) or due to kidney conditions (e.g., hydronephrosis, polycystic kidney disease, renal artery stenosis). Also associated with excess androgens or corticosteroids (e.g., Cushing's syndrome, pheochromocytoma, hyperaldosteronism, androgen therapy).
• Relative Polycythemia: An apparent increase in RBC count due to a decrease in plasma volume (hemoconcentration), without an actual increase in red cell mass. Causes include dehydration (from sweating, vomiting, diarrhea, diuretic use), severe burns, stress polycythemia, and alcoholism.
• Physiological Polycythemia of the Newborn: A temporary increase seen shortly after birth, partly due to placental transfusion and lower oxygen levels in utero.

Causes of Erythrocytosis (Polycythemia)

Category	Type	Examples
Absolute Polycythemia (Increased RBC Production)	Primary	Polycythemia Vera (Erythremia)
	Secondary (Symptomatic)	Hypoxia-driven (Appropriate EPO ↑): Chronic lung disease, congenital heart defects (cyanotic), high altitude, heavy smoking, sleep apnea (Pickwickian syndrome), abnormal hemoglobins.
		EPO-producing conditions (Inappropriate EPO ↑): Renal cell cancer, hydronephrosis, polycystic kidney disease, renal artery stenosis, hepatocellular carcinoma, cerebellar hemangioblastoma, uterine fibroids. Hormone-related: Cushing's syndrome, pheochromocytoma, hyperaldosteronism, androgen excess/therapy.
Relative Polycythemia (Decreased Plasma Volume / Hemoconcentration)		Dehydration (vomiting, diarrhea, sweating, diuretics), severe burns, stress polycythemia, alcoholism.
Mixed Polycythemia		Physiological polycythemia of the newborn.

Erythropenia / Anemia (Low RBC Count)

A reduced number of red blood cells (generally <4.0 x 10¹²/L in men, <3.7 x 10¹²/L in women) is termed erythropenia and is a primary indicator of anemia. Anemia is more formally defined by low hemoglobin or hematocrit.

Less commonly, a low RBC count can result from an increased plasma volume (hemodilution) without a true decrease in red cell mass, such as during pregnancy, excessive IV fluid administration (hyperhydration), or conditions causing high levels of plasma proteins like albumin (though hyperproteinemia itself often causes relative polycythemia).

Hemoglobin (HGB)

Hemoglobin (HGB) measures the concentration of the oxygen-carrying protein within the red blood cells, expressed typically in grams per deciliter (g/dL) or grams per liter (g/L).

• Increased HGB: Levels above the normal range (approx. >16.0-17.5 g/dL or >160-175 g/L for men, >14.0-15.5 g/dL or >140-155 g/L for women, ranges vary) usually accompany erythrocytosis/polycythemia (see Erythrocytosis table).
• Decreased HGB: Levels below the normal range (approx. <13.0-13.5 g/dL or <130-135 g/L for men, <12.0 g/dL or <120 g/L for women) are the primary laboratory definition of anemia.

The severity of anemia is often graded based on hemoglobin concentration:

• Mild anemia: HGB >10.0 g/dL or >100 g/L (approximate)
• Moderate anemia: HGB 7.0-10.0 g/dL or 70-100 g/L
• Severe anemia: HGB <7.0 g/dL or <70 g/L

Hematocrit (HCT)

Hematocrit (HCT), also known as Packed Cell Volume (PCV), represents the proportion (percentage) of the total blood volume that is occupied by red blood cells. It is influenced by both the number and the average size (MCV) of the red blood cells.

Hematocrit changes generally parallel changes in RBC count and hemoglobin:

• Increased HCT: Values above the normal range (approx. >48-52% for men, >42-47% for women) are seen in erythrocytosis/polycythemia.
• Decreased HCT: Values below the normal range (approx. <40-42% for men, <36-37% for women) are seen in anemia or states of hemodilution (increased plasma volume, e.g., pregnancy, hyperhydration, some cases of hyperproteinemia).

Red Blood Cell Indices

These are calculated values derived from the RBC count, HGB, and HCT, providing information about the average size and hemoglobin content of individual red blood cells. They are crucial for classifying anemias.

Mean Corpuscular Volume (MCV)

Mean Corpuscular Volume (MCV) indicates the average volume (size) of a single red blood cell, measured in femtoliters (fL). Normal range is typically 80-100 fL.

• Increased MCV (>100 fL) - Macrocytosis: Indicates larger-than-normal RBCs. Common causes include:
  • Megaloblastic anemias (Vitamin B12 deficiency, Folate deficiency)
  • Alcoholism
  • Liver disease
  • Hypothyroidism
  • Certain medications (e.g., hydroxyurea, some chemotherapy agents, anticonvulsants)
  • Myelodysplastic syndromes (MDS)
  • Aplastic anemia
  • Reticulocytosis (reticulocytes are larger than mature RBCs, so a high count can raise MCV)
• Decreased MCV (<80 fL) - Microcytosis: Indicates smaller-than-normal RBCs. Common causes include:
  • Iron deficiency anemia (most common cause)
  • Anemia of chronic disease/inflammation
  • Thalassemias (alpha or beta)
  • Sideroblastic anemias
  • Lead poisoning
• Normal MCV (80-100 fL) - Normocytosis: RBCs are of normal size. Anemia with normal MCV can occur in:
  • Acute blood loss (before iron stores are depleted)
  • Anemia of chronic disease (can also be microcytic)
  • Hemolysis (many types initially)
  • Aplastic anemia (can also be macrocytic)
  • Kidney disease
  • Bone marrow failure

Mean Corpuscular Hemoglobin (MCH)

Mean Corpuscular Hemoglobin (MCH) measures the average *amount* (weight) of hemoglobin contained within a single red blood cell, expressed in picograms (pg). Normal range is typically 27-33 pg.

MCH generally changes in parallel with MCV because larger cells can hold more hemoglobin and smaller cells hold less. It helps classify anemias based on hemoglobin content:

• Increased MCH (>33 pg) - Hyperchromia: Typically associated with macrocytosis (large cells holding more hemoglobin). True hyperchromia (increased concentration) is rare; high MCH usually reflects large cell size (see Macrocytosis causes under MCV).
• Decreased MCH (<27 pg) - Hypochromia: Indicates less hemoglobin per cell. Can be due to:
  • Reduced cell size (microcytosis): As seen in iron deficiency, thalassemia.
  • Impaired hemoglobin synthesis even in normal-sized cells: Sometimes seen in sideroblastic anemia or lead poisoning.
• Normal MCH (27-33 pg) - Normochromia: Found in normocytic anemias.

Mean Corpuscular Hemoglobin Concentration (MCHC)

Mean Corpuscular Hemoglobin Concentration (MCHC) measures the average *concentration* of hemoglobin within a given volume of packed red blood cells, expressed in grams per deciliter (g/dL). Normal range is typically 32-36 g/dL.

MCHC reflects the saturation of the RBC with hemoglobin, independent of cell size.

• Decreased MCHC (<32 g/dL): Indicates reduced hemoglobin concentration within the RBCs (hypochromia). Primarily seen in iron deficiency anemia and thalassemias, where hemoglobin synthesis is impaired more significantly than cell size reduction.
• Increased MCHC (>36 g/dL): True increases are rare and most strongly associated with hereditary spherocytosis (where spherical cells have less surface area for their volume, concentrating hemoglobin). Dehydration or conditions causing RBC agglutination (clumping) can sometimes cause falsely elevated MCHC readings due to instrument error. Artifactual increases can indicate issues with the sample or analyzer.
• Normal MCHC (32-36 g/dL): Seen in normocytic, normochromic anemias and most macrocytic anemias.

Red Cell Distribution Width (RDW)

Red Cell Distribution Width (RDW) measures the variation in red blood cell size (volume) within the sample, essentially quantifying the degree of anisocytosis. It is usually reported as a percentage (RDW-CV) or in femtoliters (RDW-SD).

• Increased RDW: Indicates a greater-than-normal variation in RBC sizes (anisocytosis). This is common in:
  • Iron deficiency anemia (often increases before MCV drops significantly)
  • Megaloblastic anemias (Vitamin B12/Folate deficiency)
  • Hemolytic anemias (due to presence of fragments and reticulocytes)
  • Post-hemorrhage (due to reticulocytosis)
  • After blood transfusion (mixed cell populations)
  • Myelodysplastic syndromes
• Normal RDW: Suggests RBCs are relatively uniform in size. Can be seen in anemia of chronic disease, thalassemia trait (where cells are uniformly small), aplastic anemia, or acute blood loss before significant reticulocytosis.

RDW is particularly useful in conjunction with MCV for differentiating types of anemia.

Red Blood Cell Morphology

Microscopic examination of a peripheral blood smear provides qualitative information about the appearance of red blood cells.

• Normal Morphology: Healthy RBCs appear as uniformly sized, round, biconcave discs that stain pink with a central area of pallor (normochromic, normocytic).
• Anisocytosis: Variation in RBC size (combines microcytes and macrocytes). Quantified by RDW.
• Poikilocytosis: Variation in RBC shape. Specific shapes can indicate underlying disorders.
• Anisochromia: Variation in hemoglobin content/staining intensity (combines hypochromic and normochromic cells).
• Polychromasia (Polychromatophilia): Appearance of larger, slightly bluish-tinged RBCs on the smear. These are typically young reticulocytes containing residual RNA. Increased polychromasia indicates active erythropoiesis (response to blood loss, hemolysis, or treatment for deficiency anemias). Can also be seen in dyserythropoiesis (abnormal RBC production, e.g., megaloblastic anemia, thalassemia, erythroleukemia).
• Basophilic Stippling: Presence of small, blue dots (aggregated ribosomes and RNA remnants) within the RBC cytoplasm. Associated with lead poisoning, thalassemia, sideroblastic anemia, megaloblastic anemia, MDS, and other conditions with impaired hemoglobin synthesis or dyserythropoiesis.
• Nuclear Remnants (Howell-Jolly bodies, Cabot rings):
  • Howell-Jolly bodies: Small, round, dark purple nuclear fragments. Normally removed by the spleen; their presence indicates absent or dysfunctional spleen (post-splenectomy, functional hyposplenism) or severe dyserythropoiesis (megaloblastic anemia, severe hemolysis).
  • Cabot rings: Thin, ring-like or figure-eight structures (thought to be mitotic spindle remnants). Rare, seen in severe anemias (megaloblastic, lead poisoning) and dyserythropoiesis.
• Nucleated Red Blood Cells (NRBCs / Normoblasts): Immature RBCs that still contain a nucleus. Normally confined to the bone marrow, their presence in peripheral blood indicates significant bone marrow stress or damage. Causes include severe anemia/hemolysis, hypoxia, massive hemorrhage, myeloproliferative disorders, leukemia (especially erythroleukemia), bone marrow infiltration (carcinomatosis/metastases), extramedullary hematopoiesis, and post-splenectomy state.

Specific Poikilocytes (Abnormal Shapes):

Common Poikilocytes and Associated Conditions

RBC Shape	Associated Conditions
Microspherocytes (Small, round, dense, lack central pallor)	Hereditary Spherocytosis, Autoimmune Hemolytic Anemia (AIHA), severe burns, some enzyme deficiencies.
Target Cells (Codocytes - "bullseye" appearance)	Liver disease (especially obstructive), Iron deficiency anemia, Thalassemia, Hemoglobin C disease, Post-splenectomy state.
Ovalocytes / Elliptocytes (Oval or cigar-shaped)	Hereditary Elliptocytosis/Ovalocytosis, Megaloblastic anemia, Iron deficiency anemia, Myelodysplastic syndromes, Thalassemia.
Stomatocytes (Mouth-like central pallor)	Hereditary Stomatocytosis, Liver disease/alcoholism, Rh null phenotype, artifact.
Sickle Cells (Drepanocytes - crescent/sickle-shaped)	Sickle Cell Anemia (HbSS), Sickle Cell Trait (HbAS - usually only under low oxygen), other sickle hemoglobinopathies (HbSC, HbS-beta thal).
Acanthocytes (Spur cells - irregular, thorny projections)	Severe liver disease (spur cell anemia), Abetalipoproteinemia, Neuroacanthocytosis syndromes, Malnutrition, Post-splenectomy.
Echinocytes (Burr cells - short, regular, evenly spaced projections)	Uremia (kidney failure), Pyruvate kinase deficiency, Liver disease, Artifact (common due to sample aging/preparation).
Schistocytes (Fragmented RBCs - various shapes)	Microangiopathic Hemolytic Anemias (MAHA) including: Thrombotic Thrombocytopenic Purpura (TTP), Hemolytic Uremic Syndrome (HUS), Disseminated Intravascular Coagulation (DIC), HELLP syndrome, Malignant hypertension. Also seen with mechanical heart valves, severe burns.
Teardrop Cells (Dacrocytes - teardrop shape)	Myelofibrosis, Extramedullary hematopoiesis (bone marrow infiltration by tumor/fibrosis), Thalassemia, Severe iron deficiency, Megaloblastic anemia.

Reticulocytes

Reticulocytes are young, immature red blood cells that have recently lost their nucleus but still contain residual ribosomal RNA (which stains with certain dyes like new methylene blue, allowing them to be counted specifically).

The reticulocyte count reflects the rate of red blood cell production (erythropoiesis) by the bone marrow. It is usually expressed as a percentage of total RBCs or as an absolute count.

• Increased Reticulocyte Count (Reticulocytosis): Indicates increased bone marrow activity in response to:
  • Acute blood loss (hemorrhage)
  • Hemolysis (destruction of RBCs)
  • Appropriate response to treatment for deficiency anemias (e.g., iron, B12, folate therapy)
  • Recovery from bone marrow suppression
• Decreased or Normal Reticulocyte Count in Anemia: Indicates inadequate bone marrow response (impaired erythropoiesis). Seen in:
  • Aplastic anemia / Bone marrow failure
  • Untreated iron, B12, or folate deficiency anemias
  • Anemia of chronic disease/inflammation
  • Anemia of kidney disease (due to low erythropoietin)
  • Bone marrow infiltration (leukemia, metastases)

Calculating the corrected reticulocyte count or the reticulocyte production index (RPI) adjusts for the degree of anemia and provides a better measure of marrow response.

Erythrocyte Sedimentation Rate (ESR)

The Erythrocyte Sedimentation Rate (ESR), or "sed rate," is a non-specific measure of inflammation. It measures the rate at which red blood cells settle in a vertical tube of anticoagulated blood over one hour (measured in mm/hr).

The settling rate is primarily influenced by the concentration of certain plasma proteins, particularly fibrinogen and immunoglobulins (globulins), which increase during inflammation, infection, tissue injury, or malignancy. These proteins cause RBCs to aggregate (form rouleaux), making them heavier and settle faster.

Normal ESR values vary with age and sex (typically higher in women and older adults).

Typical ESR Normal Ranges (Westergren method)

Group	Approximate Normal Range (mm/hr)
Men < 50 years	0 - 15
Men > 50 years	0 - 20
Women < 50 years	0 - 20
Women > 50 years	0 - 30

Factors affecting ESR:

Factors Influencing ESR

Factors Increasing ESR	Factors Decreasing ESR
Increased fibrinogen (inflammation, pregnancy) Increased immunoglobulins (infection, autoimmune disease, myeloma) Anemia (fewer cells, faster settling) Macrocytosis Older age Female sex Technical factors (tilted tube, high temp)	Polycythemia (high RBC count hinders settling) Abnormal RBC shapes (sickle cells, spherocytes - impair rouleaux) Microcytosis Hypofibrinogenemia Low plasma proteins (e.g., severe liver disease) High bile salt concentration Certain drugs (e.g., corticosteroids) Technical factors (delay in testing, low temp, short tube)

An elevated ESR is a non-specific indicator of inflammation, infection, autoimmune disease (rheumatoid arthritis, ankylosing spondylitis, temporal arteritis), tissue injury, or malignancy. A very low ESR can be seen in polycythemia or conditions affecting RBC shape.

The Blood Test Procedure (CBC)

• Sample Type: Whole blood collected in an anticoagulant tube (usually EDTA - purple top).
• Preparation: Generally, no fasting or special preparation is required for a standard CBC.
• Collection: Blood is drawn from a vein, typically in the arm, using standard venipuncture technique.
• Analysis: Performed using automated hematology analyzers that count cells and measure parameters. A peripheral blood smear may also be prepared for microscopic examination by a technologist or pathologist, especially if automated results show abnormalities (flags).

References

1. National Heart, Lung, and Blood Institute (NHLBI). (n.d.). Blood Tests. NIH. Retrieved from https://www.nhlbi.nih.gov/health/blood-tests
2. Lab Tests Online. (n.d.). Complete Blood Count (CBC). Retrieved from https://labtestsonline.org/tests/complete-blood-count-cbc
3. American Society of Hematology (ASH). (n.d.). Anemia. Retrieved from https://www.hematology.org/education/patients/anemia
4. Mayo Clinic Staff. (n.d.). Complete blood count (CBC). Mayo Clinic Patient Care & Health Information. Retrieved from https://www.mayoclinic.org/tests-procedures/complete-blood-count/about/pac-20384919
5. Bain, B. J., Bates, I., & Laffan, M. A. (Eds.). (2016). *Dacie and Lewis Practical Haematology* (12th ed.). Elsevier. [Note: Comprehensive hematology laboratory textbook]
6. American Association for Clinical Chemistry (AACC). (n.d.). Erythrocyte Sedimentation Rate (ESR). Retrieved from https://www.aacc.org/clinical-chemistry-trainee-council/trainee-council-in-english/pearls-of-laboratory-medicine/2020/erythrocyte-sedimentation-rate