Bone Marrow Cellularity Assessment
A bone marrow section that shows a significantly decreased amount of fat for the age of the patient would be described as hypercellular (B).
Understanding Bone Marrow Cellularity
Bone marrow cellularity refers to the ratio of hematopoietic cells to fat in the bone marrow. This ratio normally changes with age in a predictable pattern:
- In children and young adults: Higher cellularity (less fat) is normal
- In older adults: Lower cellularity (more fat) is normal
When assessing bone marrow cellularity, pathologists compare the observed ratio of hematopoietic cells to fat against age-adjusted norms.
Age-Related Changes in Bone Marrow Cellularity
Bone marrow cellularity typically follows these patterns 1:
- Under 20 years: 45-85% cellularity (mean 65%)
- 20-40 years: 40-70% cellularity (mean 55%)
- 40-60 years: 35-65% cellularity (mean 50%)
- Over 60 years: 30-60% cellularity (mean 45%)
The decline in cellularity occurs at approximately 3% per decade in adults 1. This is more stable and slower than the previously widely used formula of "100% minus the patient's age in decades."
Classification of Bone Marrow Cellularity
When evaluating bone marrow, three main classifications are used:
- Hypercellular: Increased hematopoietic cells and decreased fat for age
- Normocellular: Age-appropriate ratio of hematopoietic cells to fat
- Hypocellular: Decreased hematopoietic cells and increased fat for age
Clinical Significance
Hypercellular bone marrow (decreased fat for age) is commonly seen in:
- Myeloproliferative disorders
- Essential thrombocythemia 2
- Polycythemia vera
- Primary myelofibrosis (prefibrotic stage) 2
- Leukemias
- Reactive conditions (infections, inflammation)
In contrast, hypocellular bone marrow (increased fat for age) is seen in:
- Aplastic anemia
- Hypoplastic myelodysplastic syndromes (H-MDS) 2
- Hypoplastic acute myeloid leukemia (H-AML) 2
Diagnostic Considerations
When evaluating bone marrow cellularity:
- Always adjust for the patient's age
- Consider the distribution of hematopoietic elements (trilineage vs. single lineage predominance)
- Evaluate megakaryocyte morphology and distribution
- Look for dysplastic features
- Assess for fibrosis
Common Pitfalls to Avoid
- Failing to adjust for age: What appears hypercellular in an elderly patient might be normal in a child
- Sampling error: Bone marrow cellularity can vary within the same bone and between different bones
- Technical factors: Preparation artifacts can affect assessment of cellularity
- Confusing hypocellular AML with aplastic anemia: Immunohistochemistry (CD34, CD117) may help distinguish these entities 2
Remember that bone marrow cellularity assessment is just one component of a comprehensive bone marrow evaluation and should be interpreted in conjunction with clinical information and other laboratory findings.