Monoclonal B-cell lymphocytosis: recommendations from the Dutch Working Group on CLL for daily practice.

Monoclonal B-cell lymphocytosis (MBL) is defined by the presence of small B-cell clones in asymptomatic individuals. Usually, MBL cells are characterised by a chronic lymphocytic leukaemia (CLL) phenotype ('CLL phenotype MBL'); however, an atypical phenotype ('atypical-CLL phenotype MBL') or non-Hodgkin lymphoma phenotype ('non-CLL phenotype MBL') can be found as well. The prevalence of MBL in the general population with an age over 40 years is 3 to 5%. Subjects with MBL develop CLL requiring treatment at a rate of 1 to 2% per year. At the moment official guidelines with respect to MBL are not available in the Netherlands. On the basis of the available data, we will discuss the definitions of MBL , highlight clinical consequences and offer recommendations for daily practice. Individuals with clinically suspected MBL should undergo a complete evaluation by a haematologist. In case of CLL phenotype MBL , further annual follow-up can take place by the general practitioner. If signs of progression occur patients should be referred to a haematologist.