AbstractThe current definition of plasma cell leukemia (PCL)— ≥ 20% circulating plasma cells (CPCs) on peripheral smear and plasma cell count ≥ 2 × 10⁹/L—may be too stringent. We reviewed outcomes of 176 multiple myeloma (MM) patients diagnosed between 1971 and 2016, and who had CPCs detectable at diagnosis, to determine whether a lower threshold could be used to diagnose PCL. Median overall survival (mOS) was 1.1 years (95% CI 0.8–1.4) and was similar between patients with < 5% (n = 54, mOS = 1.4 years [0.7–2.0]), 5–19% (n = 63, mOS = 1.1 years [0.7–1.4]), and ≥ 20% CPCs (n = 59, mOS = 1.1 years [0.7–1.5], p = 0.349). As survival was similar between those with 5–19% and ≥ 20% CPCs, we stratified patients by < 5% (mOS = 1.4 years [0.7–2.0]) and ≥ 5% CPCs (mOS = 1.1 years [0.8–1.4], p = 0.154). Outcomes of those with ≥ 5% CPCs were much poorer when compared with a cohort of MM patients diagnosed between 1971 and 2016, who did not have CPCs at diagnosis (n = 9724, mOS = 4.4 yrs [4.3–4.5], p < 0.001); survival was also lower in patients diagnosed after 2001 with ≥ 5% CPCs (n = 62, mOS = 1.4 years [0.8–2.5]) compared with patients with standard risk (n = 1326, mOS = 7.5 years [7.0–8.7]) and high-risk MM (n = 381, mOS = 4.3 years [3.5–4.9], p < 0.001). We therefore propose that the definition of PCL be revised to patients with ≥ 5% CPCs on peripheral blood smear, who otherwise meet diagnostic criteria for MM.