Answer to Case on Crazy CBC Values!

Let’s deal with the second question 1st. The answer is NO! neutrophil count is only 1% but the absolute neutrophil count (ANC) is a healthy 3.3 x 10⁹/L!

Now for the first question. You might have guessed that the patient developed autoimmune hemolytic anemia as a complication of evolving CLL, with reticulocytosis-associated macrocytosis. That works, but there are two pieces of data that is tough to reconcile:

• Reticulocytes are normally 20-30% larger than mature RBCs. That means that even with 100% retics, the MCV will not typically reach 122 fL (this patient’s level). So reticulocytosis is an unlikely explanation simply based on the degree of macrocytosis.
• The MCHC falls from 34.6 to 25.3 g/dL. That’s a huge drop over 4 months, and we rarely ever see MCHCs this low. It’s true that reticulocytosis may be associated with a certain degree of hypochromia (remember, their Hb becomes more concentrated as the cell matures), but not to this low level.

This is a case of spurious macrocytosis caused by hyperleukocytosis in CLL. RBCs are counted and sized along with WBCs and platelets using the Coulter method in automated hematology analyzers.

Normally, the number of WBCs is too small to affect the RBC parameters. However, in some cases of hyperleukocytosis, the number and size of WBCs may influence the RBC indices (falsely elevated RBC count and MCV, and because MCHC = Hb/Hct, falsely low MCHC).

Appropriate corrections can be made to estimate the correct Hct, MCV and MCHC (see second graphic).

Abbreviations: RBC, red blood cell; MCV, mean cell volume; CLL, chronic lymphocytic leukemia; MCHC, mean corpuscular hemoglobin concentration; WBC, white blood cell.