Restricted κ/λ light chain ratio by flow cytometry in germinal center B cells in Hashimoto thyroiditis

To determine the diagnostic significance of the κ/λ ratio in germinal center (GC) B cells in Hashimoto thyroiditis (HT), we used 4-color flow cytometry to immunophenotype 27 samples (21 patients) of well-characterized HT. B-cell clonality was analyzed further by polymerase chain reaction (PCR) of the immunoglobulin heavy chain (IgH) and bcl-2/IgH fusion genes using DNA extracted from aspirate smears and/or paraffin-embedded tissues. By flow cytometric analysis, the CD10+ GC B cells had a higher mean ± SD κ/λ ratio than the CD10- B cells (5.1 ± 3.3 vs 2.0 ± 0.8; P < .0001, Student t test). In 18 samples (67%), CD10+ GC B cells had κ/λ ratio greater than 3.07 (the upper limit of κ/λ ratio reported in reactive nodes; range, 3.2-14.4 in the 18 cases). Cases tested by PCR showed no evidence of a clonal proliferation. None of 21 cases developed lymphoma during clinical follow-up of up to 3 years. The κ/λ ratio of CD10+ GC B cells in HT can be skewed markedly beyond that reported in reactive lymph nodes. This finding frequently is present in HT. Pathologists should be familiar with this phenomenon to prevent misdiagnosis of follicular lymphoma in patients with HT.