PARR clonality test (B-cell and T-cell) in cats and dogs

What it shows

PARR stands for PCR for antigen receptor rearrangement. Every lymphocyte, while it matures, cuts and rejoins the genes for its receptor in its own way. B cells do this in the antibody genes, T cells in the T-cell receptor genes. The test copies these gene regions and measures the length of the products.

• In a reactive population — inflammation, infection — lymphocytes come from many different cells, so the products have many lengths. The result is called polyclonal.
• In a lymphoma the cells descend from one cell, so most products have one length. The result is called clonal (monoclonal).

The two lines on the form are two reactions of the same test: B-cell PARR looks at the antibody genes, T-cell PARR at the T-cell receptor genes. The test is used when the cells under the microscope look neither clearly normal nor clearly malignant — for example, an enlarged lymph node, a raised lymphocyte count, a fluid with many small lymphocytes, or an intestinal biopsy in a cat. It is done by a specialised laboratory.

Higher than reference — what it may be a sign of

• Clonal result — fits a neoplastic population of lymphocytes such as lymphoma or lymphoid leukaemia. It supports the diagnosis made from cytology or histology; it does not replace it.
• Clonal result without cancer — some infections can cause a clonal expansion of lymphocytes. In dogs this has been described with Ehrlichia canis (T cells), Lyme disease (B cells) and Leishmania. Long-standing inflammation can also produce clonal populations: all lymphomas are clonal, but not every reactive lesion is polyclonal.
• Which line is clonal — a clonal B-cell or T-cell reaction does not reliably tell the type of lymphoma. B-cell rearrangements occur in T-cell tumours and the other way round, and some tumours are clonal in both. The cell type is determined by other methods, such as flow cytometry or immunohistochemistry.

Lower than reference

A polyclonal (negative) result fits a reactive population. It does not rule out lymphoma: one laboratory estimates the test finds about 93% of confirmed lymphomas in dogs and 91% in cats, so a small share of lymphomas test negative. A result can also be non-diagnostic — too few lymphocytes in the sample, or substances that block the reaction (mast cells can sometimes do this). In that case the vet decides whether to resample.

Look at together with

• Cytology or biopsy of the same site — the main diagnosis rests on how the cells look; clonality testing is an adjunct and is never used alone to diagnose a blood or lymphoid cancer.
• Complete blood count — lymphocytes, white blood cells and the blood smear, when the question is a raised lymphocyte count.
• Infection tests where relevant — Ehrlichia and Leishmania in dogs from areas where these occur, since they can mimic a clonal result.
• Thymidine kinase 1 — a blood marker of dividing cells; a different kind of test that answers a different question.

Units and conversion

The result has no units. Each line is reported in words: clonal (or “clonal rearrangement detected”), polyclonal (no clonal population detected), sometimes oligoclonal or non-diagnostic, often with a written comment from the laboratory. In the app these results are stored as text, one line for B cells and one for T cells.

Reference range

The expected result is on your lab’s form; for a reactive or healthy population it is “polyclonal” in both lines. A clonal result is weighed together with the cytology or biopsy, the blood count, the history and, where needed, infection tests — the final conclusion belongs to the vet and the pathologist.

Cats and dogs

Sources

1. eClinPath (Cornell University College of Veterinary Medicine). Classification tools — leukemia (PARR section).
2. Colorado State University Clinical Hematopathology Laboratory. Principles of Testing and Publications.
3. Colorado State University Clinical Hematopathology Laboratory. Sample Collection and Submission Instructions.
4. Rout ED et al. Assessment of immunoglobulin heavy chain, immunoglobulin light chain, and T-cell receptor clonality testing in the diagnosis of feline lymphoid neoplasia. Vet Clin Pathol 2019;48(Suppl 1):45–58.
5. ACVIM consensus statement guidelines on diagnosing and distinguishing low-grade neoplastic from inflammatory lymphocytic chronic enteropathies in cats. J Vet Intern Med.