Why Check B12 in Patients with Eosinophilia
Vitamin B12 levels should be checked in patients with eosinophilia because elevated B12 is a key biomarker that helps distinguish primary/clonal eosinophilic disorders from secondary/reactive eosinophilia, which has significant implications for diagnosis, treatment, and prognosis. 1
Diagnostic Value of B12 in Eosinophilia
Primary vs. Secondary Eosinophilia
- Elevated serum vitamin B12 levels are commonly observed in myeloproliferative variants of hypereosinophilia, particularly in myeloid neoplasms with PDGFRA fusion gene 1
- B12 levels are typically markedly increased in Hypereosinophilic Syndrome (HES) but normal in secondary eosinophilia 2
- This distinction is crucial as:
- Primary (clonal/neoplastic) eosinophilia requires different management than secondary causes
- Treatment approach and prognosis differ significantly between these entities
Part of Standard Workup
The NCCN guidelines specifically recommend measuring vitamin B12 levels as part of the standard laboratory testing for patients with eosinophilia, along with:
- CBC with differential
- Comprehensive metabolic panel with uric acid
- Lactate dehydrogenase
- Liver function tests
- Serum tryptase levels 1
Mechanism and Significance
Pathophysiologic Basis
- The elevation of B12 in myeloproliferative eosinophilic disorders is related to increased production of B12 binding proteins (transcobalamins I and III, also called R binders) 2
- This elevation reflects an expanded pool of both eosinophils and neutrophils in HES, providing additional evidence for classifying HES as a myeloproliferative disorder 2
- In cases of eosinophilic leukemia, extremely high levels of serum vitamin B12, unsaturated B12-binding capacity, and transcobalamin I can reach values similar to those seen in chronic myelogenous leukemia 3
Clinical Correlation
- B12 levels correlate with disease activity - studies have shown that B12 binding protein levels decrease with successful treatment of eosinophilic disorders 3
- The degree of B12 elevation may correlate with the severity of the underlying myeloproliferative process
Clinical Application
Diagnostic Algorithm
- Measure B12 levels in all patients with unexplained eosinophilia (>0.5 × 10^9/L)
- Interpret results:
- Elevated B12: Suggests primary/clonal eosinophilia, particularly myeloproliferative variants
- Normal B12: More consistent with secondary/reactive eosinophilia
- Use this information alongside other diagnostic tests:
- Bone marrow evaluation
- Genetic testing for TK fusion genes (especially PDGFRA)
- Serum tryptase levels (often elevated in myeloproliferative variants)
Pitfalls to Avoid
- Don't rely solely on B12 levels for diagnosis; they should be interpreted as part of a comprehensive evaluation
- Remember that not all cases of primary eosinophilia will have elevated B12 levels
- Be aware that vitamin B12 deficiency can coexist with eosinophilic disorders, potentially masking the expected elevation
Additional Considerations
While checking B12 is important, a comprehensive evaluation for eosinophilia should also include:
- Evaluation for secondary causes (allergies, parasitic infections, medications)
- Assessment of organ involvement and damage
- Genetic testing for specific mutations and rearrangements
- Bone marrow examination in selected cases
By checking B12 levels in patients with eosinophilia, clinicians can gain valuable diagnostic information that helps distinguish between different etiologies and guide appropriate management decisions.