LAUSR

A 75-year-old man with a history of chronic ischemic heart disease with a previously normal blood count, presented to the emergency room with fever and tachycardia. There was no hepatosplenomegaly or lymphadenopathy. An electrocardiogram showed left bundle branch block. Because of the fever the patient underwent SARS-CoV-2 RNA testing with positive result. The patient's blood count showed a WBC of 10.46 x 109/L, lymphocytes 4.51 x 109/L, hemoglobin 129 g/ L, and platelet count 233 x 109/L. D-Dimer was 659 mug/L (normal range <500) and IL6 was 76.3 pg/ml (normal range <6.4). A computed tomography scan of the chest showed bilateral interstitial infiltrates associated with multiple enlarged mediastinal lymph nodes. Following a rapid and unexpected increase of the WBC to 17.49 x 109/L with lymphocyte count of 8.37 x 109/L, a blood film and immunotyping were performed. The film showed small/medium sized lymphocytes, with a variable N: C ratio and moderately basophilic cytoplasm. Smear cells were present. About 25% of the lymphocytes showed the negative images of one to three rodshaped crystals (average 2 per cell). Some immature monocytes and neutrophils showed mild toxic granulation or abnormal nuclear shapes, consistent with COVID-19. Flow cytometric immunotyping showed an increased number of circulating B cells (93% of lymphocytes, 7.78 x 109/L) with lambda light chain restriction and expressing CD19, CD5, CD23, weak CD20, CD43, and CD200;CD10, CD79b, CD81, FMC7, and CD38 were negative. At this stage the clinical picture could not be distinguished from chronic lymphocytic leukemia (CLL). Two months later the WBC and lymphocyte count returned to normal and immuno typing showed only 0.63 x 109/L CD5-positive clonal B cells. Lymphocytes with cytoplasmic crystals were still present. A diagnosis of monoclonal B-cell lymphocytosis (MBCL) was made. Patients with CLL in whom COVID-19 led to a marked but transient increase in the lymphocyte count have been reported. In our case, COVID-19 in a patient with MBCL led to an increase in the lymphocyte count simulating CLL but follow-up indicated the correct diagnosis. We report here the observation of endocellular crystals, attributable to crystallization of immunoglobulin, in MCBL, a phenomenon previously reported in CLL.