Hypersegmented Neutrophils with Borderline Neutrophil Count: Diagnostic Workup
Check serum vitamin B12 and folate levels immediately, as hypersegmented neutrophils strongly indicate early or evolving megaloblastic anemia even when the absolute neutrophil count remains near-normal and before macrocytosis develops.
Understanding the Clinical Significance
Hypersegmented neutrophils are a critical early marker of vitamin deficiency that appears before other hematologic abnormalities become apparent:
- Hypersegmentation precedes macrocytosis: Neutrophil hypersegmentation can be present with normal hemoglobin and mean corpuscular volume (MCV), representing incipient folate or B12 deficiency 1
- High diagnostic yield: Among patients with hypersegmented neutrophils but without macrocytic anemia, approximately 77% (45 of 58 patients in one series) had documented deficiency of folate, vitamin B12, or both 2
- Persistent finding: Hypersegmentation persists for 11-14 days after initiating vitamin replacement therapy, even after neutrophil counts normalize and granulocyte folate levels are restored 3
Immediate Diagnostic Steps
Primary Laboratory Testing
Order the following tests without delay:
- Serum vitamin B12 level 2, 1
- Serum folate level 2, 1
- Complete blood count with peripheral smear review to confirm hypersegmentation and assess for macrocytosis 4
- Reticulocyte count to evaluate bone marrow response 4
Important Caveat
Be aware that approximately 10% of patients may have a bacterial inhibitor in their serum that interferes with microbiological folate assays, potentially causing falsely low results 2. If clinical suspicion remains high despite normal folate levels, consider red blood cell folate testing.
Secondary Evaluation Based on Initial Results
If Vitamin Deficiency Confirmed
- Methylmalonic acid and homocysteine levels can help distinguish B12 from folate deficiency when results are equivocal
- Intrinsic factor antibodies if B12 deficiency is present to evaluate for pernicious anemia
- Assess for malabsorption if dietary intake appears adequate
If Vitamin Levels Are Normal
Consider alternative causes of hypersegmentation:
- Renal function testing (BUN, creatinine): Uremia is a significant cause of hypersegmentation without vitamin deficiency—in one series, 7 of 13 patients with hypersegmentation and normal vitamin levels were uremic 2
- Bone marrow examination if neutropenia worsens or other cytopenias develop, to evaluate for myelodysplastic syndrome or other intrinsic marrow disorders 4, 5
Clinical Correlation
The neutrophil count of 2.1 × 10⁹/L is technically not neutropenic (neutropenia is defined as <1.5 × 10⁹/L in most populations, or <0.5 × 10⁹/L for severe neutropenia) 6, 5. However, the presence of hypersegmentation suggests:
- Early megaloblastic process: The patient may be in the early stages of vitamin deficiency before significant neutropenia or anemia develops 1
- Negative correlation: There is a significant inverse relationship between neutrophil lobe count and serum folate levels 1
- Morphologic shift: The predominant change is a shift from three-lobed to five-lobed neutrophils 1
Risk Stratification
While this patient does not currently meet criteria for severe neutropenia requiring urgent antimicrobial prophylaxis or empiric antibiotics 6, monitor for:
- Progressive neutropenia: If the count drops below 0.5 × 10⁹/L, infection risk increases substantially 4, 5
- Clinical signs of deficiency: Glossitis, neurologic symptoms (B12), or other manifestations
- Associated cytopenias: Development of anemia or thrombocytopenia
Treatment Considerations
Once vitamin deficiency is confirmed, initiate appropriate replacement therapy. Note that during early treatment of megaloblastic anemia, you may observe a paradoxical increase in hypersegmented neutrophils as the total granulocyte count rises (up to seven-fold increase in circulating hypersegmented cells by day 8) before normalization occurs 3.