Abstract AIM In this study plasma cell quantitation comparing four modalities comprising morphology of the aspirate, flow cytometry, HEtrephine section examination and bone marrow immunohistology was undertaken to determine the most sensitive technique. METHODS 70 patients with a plasma-cell dyscrasia were retrospectively analysed. Plasma-cell quantitation was performed on Romanowsky-stained aspirate slides by a 200-cell differential. Flow cytometric quantitation of plasma-cell burden was performed by identifying CD38/138 co-expressing cells. Trephine specimens were analysed after HEand CD138 staining. Statistical analysis was performed using a two-tailed paired t-test. RESULTS See Table 1. DISCUSSION Significant discrepancy was noted between the four methods for determining plasma-cell infiltration. CD138 immunohistochemical staining of paraffin embedded trephine specimens was the most sensitive modality. Flow-cytometric analysis underestimated the degree of marrow infiltration. Morphology of the aspirate sample may also underestimate plasma-cell burden as sampling may be affected by patchy marrow infiltration as well hypoplastic, fibrotic marrows or clotted specimens. Quantification of plasma cells based on HEstained trephine specimens is less sensitive, and is dependent on the technical quality of the specimen and observer experience. Immunostaining has the advantages of improved plasma-cell identification compared to HEsections and avoids the problems inherent in the analysis of aspirate samples. In conclusion, CD138 immunostaining is the most sensitive method for quantifying plasma-cell burden in the bone marrow. Two tailed Paired t-test analysis of Quantitation of Plasma Cell burden by various modalities Flow Cytometry Aspirate Hematoxylin & Eosin Immunostaining Mean % of plasma cells 5.6 18.9 23.6 29.7 Paired t-test (two tailed) Mean Difference compared to CD138 immunostaining −24.12 −11 −6.14 - 95% C.I. −30.5 to −17.8 −15.8 to −6.1 −8.34 to −3.9 - p-value p<0.0001 p<0.0001 p<0.0001 -