Quantification of CD38 expression in B-cell chronic lymphocytic leukemia (B-CLL): a comparison between antibody binding capacity (ABC) and relative median fluorescence (RMF).

We have previously shown that quantification of CD38 expression using microbeads of specific antibody binding capacity (ABC) improves the prognostic value of CD38 expression in B-cell chronic lymphocytic leukemia, particularly for Binet Stage A patients. Quantification of CD38 expression using beads is expensive, time consuming and could be difficult to implement in a routine clinical laboratory. The calculation of relative median fluorescence (RMF) using the median fluorescence intensities of the test and control samples, is even more simply and cheaply obtained by flow cytometry and could be used as an alternative way of quantifying antigen expression. The present study demonstrates that RMF is an effective prognostic indicator in B-CLL that correlates closely with ABC in predicting disease-specific survival and time to progression for all patients. RMF predicted overall survival and time to progression in all patients (P < 0.0001 for both), in Binet Stage A patients (P < 0.0001 for both) and in Stage A patients under 60 years (P = 0.0299 and P = 0.0143, respectively). ABC predicted overall survival and time to progression in all patients (P < 0.0001 for both) in Stage A patients (P = 0.0024 and P < 0.0001, respectively) and in Stage A patients under 60 (P = 0.0379 and P = 0.0032, respectively). RMF is more effective than percentage CD38 positivity > 30% or > 20% in predicting disease-specific survival in Stage A patients of all ages (CD38 < > 30%: P = 0.0853, CD38 < > 20%: P = 0.0894) and in those under 60 years old (CD38 < > 30%: P = 0.5438, CD38 < > 20%: P = 0.2872). Also, RMF is more effective in predicting time to progression of Binet Stage A patients less than 60 years (P = 0.0143), while percentage CD38 positivity of 30%, 20% or 7% did not achieve statistical significance (P = 0.1103, = 0.0547, = 0.3399, respectively). We suggest that CD38 RMF could be used clinically as an alternative to ABC to identify patients with B-CLL that are likely to progress and require early treatment.

The prognostic value of CD38 expression and its quantification in B cell chronic lymphocytic leukemia (B-CLL).

A large number of prognostic factors are available to help predict the outcome of patients who present with B-cell chronic lymphocytic leukemia (B-CLL). These include clinical stage, leukemic cell morphology, lymphocyte doubling time, the pattern of infiltration in bone marrow trephine biopsies, cytogenetic abnormalities, p53 function and serum factors such as beta-2 microglobulin. Two recently described major prognostic factors are immunoglobulin heavy chain variable region (IgVH) mutation status and cell membrane expression of CD38. These are both highly significant independent prognostic factors, but are not closely correlated. Whereas IgVH mutational status is a time consuming and demanding technique, only available in a limited number of centres, CD38 expression by flow cytometry is relatively simple and rapidly obtained in most diagnostic laboratories. The predictive value of CD38 expression is enhanced by measurement of antigen density in terms of antibody binding capacity (ABC) rather than as the percentage of cells expressing the antigen. ABC correlates closely with relative median fluorescence (RMF), a parameter which is even more simply and cheaply obtained by flow cytometry. One of these methods of determining CD38 expression should be employed routinely. Recent work suggests that membrane ZAP-70 expression determined by flow cytometry will prove to be an accurate proxy for IgVH mutational status and this assay will be within the reach of any laboratory skilled in flow cytometry. The combination of ZAP-70 expression, CD38 antigen density, p53 function and the concentration of serum factors such as soluble CD23, is likely to provide extremely accurate prognostic information in future studies. This will assist in identifying Stage A patients who may benfit from early and/or more intensive treatment, as well as Stage B and C patients who may require alternative treatment strategies at the outset.