# Prevalence of Rheumatoid Factor and Anti-citrullinated Protein Antibodies in Patients With Post-treatment Lyme Disease

## TL;DR

This study finds that rheumatoid factor and anti-citrullinated protein antibodies are uncommon in patients with post-treatment Lyme disease, suggesting they are not useful for diagnosing joint pain in these patients.

## Contribution

The study reports the prevalence of rheumatoid factor and anti-citrullinated protein antibodies in a well-characterized cohort of post-treatment Lyme disease patients.

## Key findings

- Rheumatoid factor was positive in 3.8% of patients and anti-citrullinated protein antibodies in 4.8%.
- Only 7.2% of patients had a positive rheumatoid factor or anti-citrullinated protein antibody test at any time point.
- The prevalence of these antibodies was similar to general population rates, indicating a lack of inflammatory arthritis in post-treatment Lyme disease.

## Abstract

Post-treatment Lyme disease (PTLD) occurs in a portion of patients after initial antibiotic treatment of Lyme disease (LD) and is often characterized by arthralgia without synovitis. Rheumatoid factor (RF) and anti-citrullinated protein antibodies (ACPA) are often used to assess joint pain in this setting; however, their clinical utility remains unknown. Our objective was to define the frequency of these autoantibodies in a large cohort of carefully characterized patients with PTLD meeting a research case definition and to determine the clinical implications of these tests. RF and ACPA were tested as indicated clinically and abstracted by chart review. The prevalence of antibodies and their relationship to symptoms were examined. Of the 167 patients included in the analysis, RF status was documented at least once for 78.4% (131 of 167), and ACPA status was available at least once for 88.0% (147 of 167). RF was positive in 3.8% (five of 131), and ACPA was positive in 4.8% (seven of 147) at least at one time point. A total of 7.2% (12 of 167) patients were found to have a positive RF or ACPA test at least at one time point. There was no difference in the proportion of patients with RF and/or ACPA based on the initial presenting manifestations of their LD, nor the symptoms of PTLD at later evaluation; however, the small sample size may limit our ability to detect these clinical differences. We found a low prevalence of RF and ACPA in this study, similar to the known rates in the general population. This reflects the lack of inflammatory arthritis in this population with clinically defined PTLD and arthralgia only.