Clonality and malignancy. PCR assays for the diagnosis of clonal B- and T-cell proliferations: potentials and pitfalls.

The discrimination between lymphomas and reactive lymphoproliferations is difficult or even impossible in a proportion of cases by means of histology and additional immunhistochemistry alone. Therefore, the analysis of the clonality of the rearranged immunoglobulin genes and T-cell receptor genes, respectively, is often employed as an additional diagnostic tool. This analysis is based on the fact that the tumor cells of lymphomas harbor identically (clonally) rearranged antigen receptor genes whereas reactive lymphoproliferations consist of cells with differently (polyclonally or oligoclonally) rearranged antigen receptor genes. Initially Southern blot analysis was used to determine the presence of clonal lymphoid cell populations. Meanwhile polymerase chain reaction (PCR) is the method of choice because of its higher sensitivity and its applicability to DNA extracted from formalin-fixed tissues. Although this PCR approach appears to be simple, there are many pitfalls, which can lead to false negative or false positive results. In a diagnostic setting, these false clonality results could give rise to a wrong final diagnosis and, as a consequence, to the performance of an inadequate therapy. The reasons for the non-detectability of clonal B-cell or T-cell populations or for the detection of false clonality (e.g. clonal rearrangement pattern in the absence of a lymphoma) are manifold. In the following the most frequent reasons are discussed and recommendations are given to avoid false negative and false positive results.