4. Progressive lymphocytosis with an increase of more than 50% over a 2-month period or a lymphocyte doubling time (LDT) of less than 6 months (which may be extrapolated). Lymphocyte doubling time can be obtained by linear regression extrapolation of ALCs obtained at intervals of 2 weeks over an observation period of 2 to 3 months. For patients with initial blood lymphocyte counts of less than 30 x 109/L (30,000/μL), LDT should not be used as a single parameter to define indication for treatment. In addition, factors contributing to lymphocytosis or lymphadenopathy other than CLL (eg, infections) should be excluded.