Stable MCV and MCH Do Not Rule Out MDS in This Clinical Context
In a patient with known MDS receiving tocilizumab who has lost 50 pounds over 5 months, stable MCH (35 pg) and MCV (101 fL) over 13 months do NOT argue against the MDS diagnosis. The macrocytosis (MCV 101) is actually consistent with MDS, and stability of these parameters does not exclude disease progression or the underlying diagnosis.
Why Stable Red Cell Indices Don't Exclude MDS
MDS characteristically presents with macrocytic anemia 1. Your MCV of 101 fL falls into the macrocytic range, which is the typical pattern seen in MDS patients. The guidelines explicitly state that patients with MDS have "mostly anaemia (usually macrocytic)" 1.
The stability of MCV and MCH over 13 months is not a diagnostic criterion for or against MDS. The diagnosis of MDS requires 1, 2:
- Persistent cytopenias (≥4-6 months)
- Dysplasia in ≥10% of cells in one or more bone marrow lineages
- 5-19% blasts, OR
- MDS-associated cytogenetic abnormalities
None of these criteria depend on changing red cell indices over time.
The Prognostic Significance of Your MCV
Interestingly, your MCV of 101 fL may actually carry prognostic information. Research shows that MCV ≤100 fL is associated with worse prognosis in MDS patients with <5% bone marrow blasts 3, 4, 5. Your MCV of 101 fL places you just above this threshold, which paradoxically may indicate a relatively better prognosis within the MDS spectrum if your blast count is low.
In patients with abnormal karyotypes, those with macrocytosis (MCV >100 fL) had median survival of 31 months versus 16.5 months for those with MCV <100 fL 4.
Critical Concern: The 50-Pound Weight Loss
The dramatic weight loss of 50 pounds over 5 months is the red flag here, not the stable red cell indices. This requires urgent investigation for:
- MDS progression to higher-risk disease or AML transformation - which can occur despite stable MCV/MCH
- Tocilizumab-related complications - though the drug label 6 doesn't specifically list macrocytosis as a side effect
- Concurrent malignancy or infection - immunosuppression from tocilizumab increases risk
- Nutritional deficiencies - though your MCV argues against B12/folate deficiency as the primary cause
What You Need Now
Immediate bone marrow evaluation is warranted given the significant weight loss in a patient with known MDS. The evaluation should include 1:
- Bone marrow aspirate and biopsy with blast count (review 500 cells)
- Cytogenetic analysis
- Flow cytometry for aberrant immunophenotype
- Molecular testing for MDS-related mutations (particularly TP53, SF3B1, DNMT3A, ASXL1, TET2)
- Iron staining (Perls stain) to evaluate for ring sideroblasts
Check for other causes of weight loss:
- Complete metabolic panel, LDH, ferritin (prognostic in MDS) 1
- Exclude infection given tocilizumab immunosuppression
- CT imaging if not recently performed
Common Pitfall to Avoid
Do not be falsely reassured by stable CBC parameters in a patient with known MDS who develops new concerning symptoms. MDS can progress to higher-risk disease or transform to AML while maintaining relatively stable red cell indices. The weight loss suggests something has changed clinically, regardless of the MCV/MCH stability.
The diagnosis of MDS was presumably already established through bone marrow examination showing dysplasia, cytogenetic abnormalities, or molecular markers 1, 2. Stable red cell indices over time neither confirm nor refute that original diagnosis—they simply indicate that this particular aspect of the disease has remained constant while other features may be evolving.