Definition of Anemia
Anemia is defined as a hemoglobin concentration below the lower limit of normal for the relevant population and laboratory, with the WHO defining this as <130 g/L in men over 15 years and <120 g/L in non-pregnant women over 15 years. 1
Standard Diagnostic Thresholds
The most widely accepted definitions come from established guidelines:
- Men ≥18 years: Hemoglobin <135 g/L (13.5 g/dL) 2
- Non-pregnant women ≥18 years: Hemoglobin <120 g/L (12.0 g/dL) 1, 2
- Pregnant women (2nd and 3rd trimester): Hemoglobin <110 g/L (11.0 g/dL) 1
These thresholds represent values below the 5th percentile for the general population and are endorsed by both the WHO and major clinical practice guidelines 1, 2.
Important Population-Specific Considerations
The standard WHO cutoffs may not apply to certain populations, and clinical judgment is required for: 2
- Pregnant or menstruating women
- Individuals living at high altitude
- Smokers
- Men aged ≥70 years
- Non-Caucasian populations
- Patients with chronic lung disease
- Individuals with hemoglobinopathies
Measurement Preference
Hemoglobin is strongly preferred over hematocrit for diagnosing anemia because it demonstrates superior reproducibility across laboratories, lower coefficients of variation, and is unaffected by sample storage time or patient-specific variables like serum glucose 2.
Clinical Context and Severity
While these thresholds define anemia, the clinical significance varies by severity: 1
- Investigation for underlying pathology (particularly GI malignancy) should be considered at any level of anemia when iron deficiency is present
- More severe anemia (<110 g/L in men, <100 g/L in non-menstruating women) warrants urgent investigation
- Even mild anemia (10-12 g/dL) in elderly patients is associated with increased morbidity and mortality 3, 4
Controversy Regarding Cutoff Values
Recent research suggests the WHO cutoffs may be higher than physiologically necessary. A 2021 multinational study found pooled 5th percentile values of 9.65 g/dL in children and 10.81 g/dL in women among healthy populations, with inflection points for increased erythropoiesis at 9.61 g/dL and 11.01 g/dL respectively 5. However, clinical practice should continue using the established WHO/guideline thresholds 1, 2, as these are validated for identifying clinically significant anemia associated with adverse outcomes and are universally accepted for triggering appropriate investigation and management.
Morphological Classification
Once anemia is diagnosed, classification by mean corpuscular volume (MCV) and mean corpuscular hemoglobin (MCH) helps identify etiology, with MCH being more reliable as it is less dependent on storage conditions and counting equipment 1.