Biological properties of bone marrow plasma cells influence their recovery in aspirate specimens: impact on classification of plasma cell disorders and potential bias to evaluation of treatment response.

Methods to estimate bone marrow plasma cells (BMPC) basically include histopathology, cytomorphology, and flow cytometry. The present study compares the outcomes of these methods with special focus on the impact of BMPC-specific characteristics on their recovery by either method. Laboratory reports of diagnostic samples from 238 consecutive patients with suspected or known plasma cell disease were retrospectively analyzed. The median (IQR) proportion of BMPC was 30.0% (15.0-70.0%) by histological review (hBMPC), 7.0% (2.0-16.0%) by smear review (sBMPC), and 3.0% (0.8-10.0%) by flow cytometry (fBMPC). The disparity of results between core biopsy and aspirate smear was enhanced in case of poor quality of the smear, increased BM fiber content, higher grade cell atypia, expression of CD56 (all P < 0.0001), the number of cytogenetic aberrations (P = 0.0002), and abnormalities of the MYC gene (P = 0.0002). Conversely, expression of CD19 and a non-clonal plasma cell phenotype were associated with a lower difference between hBMPC and sBMPC (both P < 0.0001). The disparity between the percentages of sBMPC and fBMPC was associated with the quality of the smear (P = 0.0007) and expression of CD56 (P < 0.0001). Our results suggest that the recovery of BMPC in aspirate specimens not only is a matter of sampling quality but also depends on biological cell properties. Aspiration failure due to malignant type features of BMPC may lead to misclassification of plasma cell disorders and represent a bias for the detection of minimal residual disease after therapy.

Lysis matters: red cell lysis with FACS Lyse affects the flow cytometric enumeration of circulating leukemic blasts.

The whole blood lysis method has become a standard procedure to remove red cells prior to immunophenotypic analysis of leukocytes. In the present study we investigated the influence of four different lysis protocols on the flow cytometric recovery of leukemic blasts. 32 blast cells containing blood samples were stained with anti-CD45 and anti-CD34 monoclonal antibody combinations. Red cell lysis was performed with FACS Lysing Solution and BD PharmLyse™ (Becton Dickinson and Company BD Biosciences, San Jose, CA; n=32) as well as Optilyse C and IOTest 3 (Immunotech SAS, Marseille; n=15 out of 32). Flow cytometric enumeration of blasts was performed on a FACS-Canto flow cytometer. The percentage of blasts after treatment with FACS Lyse was significantly smaller than after PharmLyse™ (p<0.0001), Optilyse C (p<0.0001), or IOTest 3 (p<0.0001), respectively. The difference between PharmLyse™ and Optilyse C (p=0.93), PharmLyse™ and IOTest 3 (p=0.31), and Optilyse C and IOTest 3 (p=0.34) was not significant. These results emphasize the importance of harmonization of red cell lysis protocols for the application of flow cytometry in hematological neoplasms.