Monoclonal B lymphocytosis

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Monoclonal B-lymphocytosis (MBL) as a distinct entity recognised in the WHO Classification and used to denote the presence of abnormal clonal B-cells in blood, but with counts that do not meet the diagnostic criteria for CLL or other LPD. This is an important distinction since affected individauals are not regarded as having a lymphoproliferative disorder, although they may experience some symptoms, and the disease may progress.

The diagnosis is suggested from flow cytometry of blood. The report should record the count of abnormal lymphocytes and refer to the phenotype of the cells identifying them as either "CLL phenotype" or "non-CLL phenotype" based on the flow cytometry profile, but it is important also that the report reflects the need for additional clinical confirmation (see below).

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Diagnosis is made from blood assessment by flow cytometry and is based on the following criteria that are both flow cytometric and clinical:

1. Flow cytometry defined features:

• Evidence that there is a monoclonal population of B lymphocytes (this can be based on their light chain retriction or abnormal phenotype features)

• A profile of by flow cytometry that suggests either CLL or other LPD, based on the abnormal gated population that may be a minority of cells

• A count of abnormal cells that is <5x10⁹ for the abnormal population (usually calculated from the gated abnormal poulation and the absolute lymphocyte count

AND

2. Clinical evidence that there is no detectable evidence of tissue disease or its effects:

• The abnormal count should be present for >3 months

• No evidence of lymph node enlargment or other significant tissue involvements by the abnormal cells

• No other features suggesting an active B-lymphoproliferative disease or lymphoma

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