The purpose of the clonality assay (PCR for antigen receptor rearrangements, PARR) is to determine whether a population of lymphocytes are reactive or neoplastic. The assay uses the polymerase chain reaction (PCR) to amplify the immunoglobulin gene (for B cells) or the T-cell receptor gene (for T cells). There are innumerable possible PCR product sizes when a mixed population of lymphocytes is tested. When the amplified DNA products are the same size, the lymphocyte population is derived from the same precursor and is suggestive of a neoplastic process, although chronic E. canis infection also can cause this and should be investigated. PARR can also indicate whether a clonal population of lymphocytes are B-cells or T-cells.

Clonality assay is offered in cats but appears to be less sensitive for identifying neoplastic lymphocyte populations than in dogs.

Samples can include either: Blood in LTT; blood smears; lymph node or bone marrow aspirates (on slide or in LTT); or body cavity effusions. Cytological preparations that have been stained can be used, but formalin-fixed samples cannot be used for the PARR.

Lymph nodes were evaluated from 72 dogs with lymphadenopathy. Results of PARR indicated 85% sensitivity for lymphoid malignancy with 92% specificity (i.e. 8% false positive). The false-positive cases included dogs with E. canis (2 cases) ; Lyme disease, blastomycosis, and RMSF 1 case each).

Evaluation of peripheral blood by PARR in 86 dogs with confirmed diagnosis of multicentric LSA by PARR revealed that 85% of dogs had peripheral blood involvement. In the literature, when evaluated by blood smears, 20-60% of dogs with multicentric LSA have peripheral blood involvement. The broad range of findings here indicates the inherent subjectivity of visual blood smear analysis.

Evaluation by PARR of ~ 20 dogs with E. canis seropositivity and peripheral blood lymphocytosis showed that a few of them have clonal populations of lymphocytes.