Definition of Anemia
Anemia is defined by the World Health Organization as hemoglobin <13.0 g/dL in adult men and <12.0 g/dL in non-pregnant adult women, with specific thresholds varying by age and pregnancy status. 1, 2
Standard Hemoglobin Thresholds by Population
Adults
- Men: Hemoglobin <13.0 g/dL 1, 2, 3
- Non-pregnant women: Hemoglobin <12.0 g/dL 1, 2, 3
- Pregnant women: Hemoglobin <11.0 g/dL 1, 2
Pediatric Populations
- Children 6 months to 5 years: Hemoglobin <11.0 g/dL 1, 2
- Children 5 to 11 years: Hemoglobin <11.5 g/dL 1, 2
- Children 12 to 13 years: Hemoglobin <12.0 g/dL 1, 2
Severity Classification
The WHO establishes severity categories specifically for women ≥15 years based on hemoglobin concentrations 2:
Critical Clinical Considerations
These WHO thresholds apply regardless of age in elderly patients, as low hemoglobin in older adults typically reflects underlying pathology rather than normal aging. 2, 3 The American Journal of Kidney Diseases emphasizes that anemia should never be considered a normal consequence of aging 3.
Special Population Adjustments
In chronic kidney disease, investigation of anemia is recommended at hemoglobin <13.5 g/dL in men and <12.0 g/dL in women 2. For cardiovascular surgery and perioperative optimization, maintaining hemoglobin ≥13.0 g/dL in all patients regardless of sex improves outcomes 2.
Diagnostic Workup After Identifying Anemia
Once anemia is confirmed by WHO criteria, the minimum evaluation includes 1, 2:
- Red blood cell indices: MCV, MCH, RDW to classify as microcytic, normocytic, or macrocytic 1, 2
- Reticulocyte count: Distinguishes regenerative from non-regenerative anemia 1, 3
- Iron studies: Serum ferritin, transferrin saturation 1, 2
- Inflammatory markers: CRP concentration 1, 2
Iron Deficiency Diagnostic Criteria
The diagnostic criteria for iron deficiency depend critically on the presence of inflammation 1:
- Without inflammation: Serum ferritin <30 μg/L indicates iron deficiency 1
- With inflammation: Serum ferritin up to 100 μg/L may still represent iron deficiency 1
- Ferritin 30-100 μg/L with inflammation: Suggests combined true iron deficiency and anemia of chronic disease 1
Anemia of Chronic Disease Criteria
In the presence of inflammation, anemia of chronic disease is diagnosed by serum ferritin >100 μg/L and transferrin saturation <20% 1.
Important Caveats
The current WHO thresholds were established in 1968 based predominantly on Caucasian populations in Europe and North America, and normal hemoglobin distribution varies with ethnicity 2, 4. However, these remain the internationally accepted standards for clinical practice 1, 2.
Even mild anemia by WHO criteria is associated with increased perioperative morbidity and mortality, respiratory complications, septic complications, thromboembolic events, myocardial injury, cardiovascular events, and cognitive decline in elderly individuals 2.