Objectives: To determine the incidence of serious myelotoxicity from intravenous cyclophosphamide (IC) in systemic lupus erythematosus (SLE).
Methods: In a retrospective study, white blood cell (WBC) counts with differential and platelet counts were determined in 92 SLE patients (96 courses) given 1623 doses of IC.
Results: Only one patient developed a total leucocyte count <1000/mm3, one developed a neutrophil count <500/mm3, two had a lymphocyte count <100/mm3 and no patients had platelet counts <50000/mm3 during follow-up. The risk of a neutrophil count <500/mm3 was 0.06 per 100 visits [95% confidence interval (CI) 0.00-0.34]. Two patients discontinued IC due to neutropenia [rate of 0.12 per 100 doses (95% CI 0.01-0.44)]. No clinical consequences were recorded in conjunction with low blood cell counts. In multivariate models, both the cumulative number of IC doses and European Consensus Lupus Activity Measurement (ECLAM) score affected neutrophil and lymphocyte counts adversely. For neutrophils, lowering the ECLAM score by 1 point counteracted four additional doses of IC after adjusting for steroid dose.
Conclusions: IC and SLE disease activity have independent effects in lowering white blood cell counts, but serious myelotoxicity of IC is uncommon.