Expression strength key

Expression strength effectively reflects how strongly a molecule is expressed by neoplastic cells based on the intensity of the observed fluorescence.

It can be helpful in flow cytometric diagnosis: e.g. the characteristically weak expression of CD79b by CLL cells, or the characteristically strong expression of CD25 by the cells of HCL

It is however associated with some difficulties since antibodies and fluorochromes can differ between laboratories and the judgment is often subjective based on the compartor used. However, as the concept is useful we provide a general statement on expected fluorescence intensity that can be expected for each disorder; it is important to be aware of the lab-lab variability and assessment when used for interpretation.

This group recommended descriptions of antigen distribution are "negative" for antigens not expressed, "positive" for antigens expressed, or "partially expressed" for antigens that are expressed in a subset of the population in question. The reference for distribution should be an appropriate negative control population. The recommended descriptions of antibody fluorescence intensity are "dim" for a uniformly positive population with lower mean fluorescence intensity than a positive normal cell population, "bright" for a uniformly positive population with higher intensity than the control, and "heterogeneous" for antigens that show variable expression or a spectrum of expression intensity, as opposed to a tight uniform cluster of events. The reference for intensity is the closest normal hematolymphoid population. For example, in B-cell chronic lymphocytic leukemia, the appropriate intensity reference population for CD19, CD20, CD22, CD23, and kappa/lambda antigen distribution would be normal polyclonal B lymphocytes. This approach is similar to that described in the instructions for leukemia/lymphoma flow cytometry proficiency testing published by the College of American Pathologists.