Measuring CD19 Counts in Patients on IV Rituximab
Measuring CD19 counts in patients on IV rituximab is NOT recommended to guide re-dosing decisions, as disease flares can occur despite complete B-cell depletion and negative CD19 counts do not reliably predict clinical relapse. 1
Primary Guideline Recommendation
The 2021 American College of Rheumatology/Vasculitis Foundation conditionally recommends scheduled re-dosing of rituximab over using CD19+ B cell counts (or ANCA titers) to guide re-dosing in patients with GPA/MPA receiving rituximab for remission maintenance. 1
Evidence Behind This Recommendation
In a randomized trial, patients who received rituximab based on changes in CD19+ B cell counts and/or ANCA titers had similar relapse rates as those receiving scheduled dosing, though the study had small sample size and wide confidence intervals. 1
The critical limitation is that flares can occur when patients have complete CD19+ B cell depletion and/or when ANCA tests are negative, meaning these biomarkers do not accurately indicate disease flare potential. 1
When CD19 Monitoring May Have Limited Utility
Pharmacodynamic Monitoring (Not for Dosing Decisions)
CD19+ B cells typically deplete to <10 cells/µL following the first two infusions of rituximab, remaining at that level in most patients (84%) through Month 6. 2
By Month 12, the majority of patients (81%) show signs of B-cell return with counts >10 cells/µL, and by Month 18, most patients (87%) have counts >10 cells/µL. 2
In rheumatoid arthritis patients, the majority demonstrate near complete depletion (CD19 counts below 20 cells/µL) within 2 weeks after the first rituximab dose. 2
Predictive Value for Relapse (Research Context Only)
While not recommended for guiding re-dosing, research suggests CD19 monitoring may have some predictive value:
In IgG4-related disease, complete depletion of CD19+ B cells at 6 months after rituximab was associated with lower relapse rates (17% vs 60%, P=0.02) and longer median relapse-free survival (38 vs 19 months, P=0.02). 3
In rheumatoid arthritis, CD19+ B lymphocyte repopulation preceded clinical relapse by approximately 4 months, though this was in a research setting. 4
In SLE, profound B cell depletion (to <5 CD19+ B cells/µL) was associated with significant clinical improvement, though this does not translate to using CD19 counts for re-dosing decisions. 5
Common Pitfalls to Avoid
Do not delay scheduled rituximab re-dosing based on persistently low or undetectable CD19 counts, as disease can flare despite complete B-cell depletion. 1
Do not assume that B-cell repopulation automatically indicates need for re-treatment, as the timing of repopulation does not reliably correlate with clinical relapse across all conditions. 1
Recognize that when B cells do return after rituximab, they typically have an immature, naive phenotype (IgD+, CD27-, high CD38 and CD24 expression), which may not reflect functional immune reconstitution. 6, 7
Practical Clinical Approach
Use scheduled re-dosing protocols rather than biomarker-guided approaches:
For GPA/MPA maintenance: Rituximab 1,000 mg IV every 6 months or 375 mg/m² IV once weekly for 4 weeks, repeated at scheduled intervals. 1, 8
Monitor clinical disease activity, symptoms, and organ function rather than CD19 counts to assess treatment response and need for continuation. 1
If CD19 monitoring is performed for research or institutional protocols, interpret results cautiously and do not use as sole determinant for re-dosing decisions. 1