A 91-year-old woman presented with asthenia and fever. Blood count showed hemoglobin of 83 g/L, white blood cell count of 470 × 10⁹/L, and platelet count of 185 × 10⁹/L. The anemia was macrocytic (mean corpuscular volume [MCV]: 107.9 fL) and hypochromic (mean corpuscular hemoglobin concentration [MCHC]: 27.6 g/dL), with a red blood cell (RBC) count of 3.1 × 10¹²/L. Peripheral blood analysis was consistent with the diagnosis of chronic lymphocytic leukemia (CLL), thanks to typical cytology (panel A; original magnification ×40, May-Grünwald Giemsa stain) and a CLL score of 5/5. RBC graph by impedance (panel B) shows an atypical population of cells, counted as RBC by the analyzer, normally absent in healthy patients (panel C). This population of CLL lymphocytes are small-sized cells, slightly bigger than erythrocytes. RBC count can be corrected by optical measurement (2.7 T × 10¹²/L) or by subtracting WBC from RBC (2.63 × 10¹²/L). It is consequently possible to determine the RBC most-frequent volume (the real MCV: 99.7 fL) and to recalculate the MCHC (33 g/dL). Furthermore, it is known that major hyperleukocytosis leads to an overestimation of hemoglobin measurement by spectrophotometry.

This case illustrates a cause of false macrocytic anemia appearing when RBC and platelets are counted by impedance technique on the same channel. This method relies only on cell size, and although unexceptional, this situation must be known to correctly characterize anemia.