What defines anemia in terms of red blood cell measurements?

Anemia Definition and Classification

Anemia is defined as a reduction in hemoglobin concentration, red blood cell count, or hematocrit below normal levels. 1

Key Measurements for Diagnosing Anemia

Anemia is diagnosed based on the following red blood cell measurements:

• Primary measurements used for diagnosis:
  • Hemoglobin concentration (Hb)
  • Hematocrit (Hct)
  • Red blood cell count (RBC)

These three parameters are the major red blood cell measurements used to define anemia, with hemoglobin being the most direct and sensitive measure 1.

Diagnostic Thresholds

The World Health Organization (WHO) defines anemia as:

• Men (>15 years): Hemoglobin <130 g/L
• Non-pregnant women (>15 years): Hemoglobin <120 g/L
• Pregnant women (2nd and 3rd trimester): Hemoglobin <110 g/L 1

Severity Classification

Anemia is classified by severity as:

• Mild anemia: Hb 10.0-11.9 g/dL
• Moderate anemia: Hb 8.0-9.9 g/dL
• Severe anemia: Hb <8.0 g/dL 1, 2

The National Cancer Institute's Common Toxicity Criteria grades anemia as:

• Grade 0: Hb within normal limits
• Grade 1: Hb lower normal limit to 10.0 g/dL
• Grade 2: Hb 8.0 to <10.0 g/dL
• Grade 3: Hb 6.5 to <8.0 g/dL
• Grade 4: Hb <6.5 g/dL
• Grade 5: Death 1

Additional Diagnostic Parameters

While hemoglobin, hematocrit, and RBC count are the primary measurements for diagnosing anemia, a complete evaluation should include:

• Red cell indices:

  • Mean cell volume (MCV) - helps classify anemia as microcytic, normocytic, or macrocytic
  • Mean cell hemoglobin (MCH) - may be more reliable than MCV for detecting iron deficiency 1

• Iron studies:

  • Serum ferritin - single most useful marker for iron deficiency anemia
  • Transferrin saturation - helpful when false-normal ferritin is suspected 1

• Other parameters:

  • Reticulocyte count - evaluates bone marrow response to anemia
  • Vitamin B12 and folate levels - to rule out deficiencies
  • Peripheral blood smear - to assess red cell morphology 1, 2

Clinical Considerations

• Hemoglobin is preferred over hematocrit for diagnosing anemia as it is more reproducible across laboratories and less affected by storage time or patient-specific variables 1
• The normal range for hemoglobin varies between different populations, so it's reasonable to use the lower limit of the normal range for the laboratory performing the test 1
• Special populations may require different thresholds (pregnant women, elderly, those living at high altitude, smokers, and those with chronic lung disease) 1

Common Pitfalls

• Assuming all anemia is due to iron deficiency without proper evaluation
• Failing to recognize that anemia may be multifactorial
• Not considering that normal hemoglobin ranges vary with age, sex, ethnicity, and physiological status
• Overlooking the need to evaluate all three primary parameters (Hb, Hct, RBC count) when diagnosing anemia
• Relying solely on automated cell counter results without clinical correlation

Remember that anemia is not an independent phenomenon but often a manifestation of underlying pathology that requires thorough investigation to determine the cause.