Next Steps in Workup for Atypical Mast Cells with No CD2 Expression
The next step in the workup for a patient with atypical mast cells and no CD2 expression should be testing for KIT D816V mutation using a highly sensitive assay such as allele-specific oligonucleotide quantitative reverse transcriptase polymerase chain reaction (ASO-qPCR). 1
Diagnostic Algorithm for Atypical Mast Cells with Negative CD2
Molecular Testing
- Perform KIT D816V mutation testing using ASO-qPCR on bone marrow or peripheral blood 1
- If KIT D816V is negative, evaluate for alternative codon 816 mutations (D816H, D816Y, etc.) through sequencing of codon 17 or peptide nucleic acid (PNA)-mediated PCR 1
- If no mutation is found at codon 816, consider sequencing the whole KIT coding sequence by next-generation sequencing (NGS) 1
Additional Immunophenotyping
- Confirm CD25 expression, which is more sensitive than CD2 for diagnosing systemic mastocytosis (SM) 1, 2
- Consider testing for CD30 expression, which can be helpful in cases where CD25 is negative 1
- CD30 expression along with absence of CD2 may be useful in diagnosing well-differentiated systemic mastocytosis (WDSM) 1
Bone Marrow Evaluation
- Assess mast cell morphology (spindle-shaped, well-differentiated, or immature) 1
- Determine the percentage of abnormal mast cells out of total mast cells 1
- Evaluate pattern of involvement (multifocal dense infiltrates or primarily interstitial pattern) 1
Interpretation of CD2 Negativity
CD2 negativity in mast cells does not rule out systemic mastocytosis, as:
- CD25 is a more sensitive marker than CD2 for SM diagnosis 1, 2
- CD2 is only expressed in about 50-60% of mast cells in indolent SM and is often not expressed in advanced SM 1, 3
- Recent studies show CD2 expression in 78% of SM cases overall, with significantly less expression (38%) in SM with associated hematologic neoplasm (SM-AHN) compared to pure SM (91%) 3
Additional Testing Considerations
- If there is primarily interstitial pattern of mast cells with peripheral blood eosinophilia and KIT D816V negativity, test for FIP1L1-PDGFRA fusion gene 1
- Perform myeloid mutation panel testing to detect prognostically relevant mutations in genes such as TET2, SRSF2, CBL, ASXL1, RUNX1, JAK2, and/or RAS 1
- Consider chromosome analysis, especially if an associated hematologic neoplasm (AHN) is suspected 1
Clinical Implications
- CD2-negative but CD25-positive mast cells still strongly support a diagnosis of SM 2, 4
- The absence of CD2 expression may be associated with SM-AHN and should prompt careful evaluation for an associated hematologic neoplasm 3
- Flow cytometric immunophenotyping has shown 100% sensitivity and 91-99% specificity for SM diagnosis, even with variable CD2 expression 2, 4
Remember that while CD2 negativity is notable, the WHO criterion is "CD25 and/or CD2" expression, with evidence suggesting that CD25 alone is actually more informative for diagnosis 2.