Serum procalcitonin (PCT) level is a more specific marker for detection of bacterial infection in patients with rheumatoid arthritis (RA) than C-reactive protein (CRP) level, erythrocyte sedimentation rate (ESR), or white blood cell (WBC) count. High PCT levels (0.5 ng/mL or higher) strongly suggest bacterial infection, but a PCT level lower than 0.5 ng/mL does not rule it out.
Sato and colleagues studied 118 patients who experienced an RA flare, a noninfectious complication of RA or its treatment, a nonbacterial infection, or a bacterial infection. The PCT level was higher in the bacterial infection group than in the other 3 groups (25.8% vs 0% to 4.3%), and the difference was significant among groups. Conversely, no statistically significant difference was observed among the groups with a CRP concentration of 0.3 mg/dL or higher, WBC count higher than 8500/μL, or ESR higher than 15 mm/h. The specificity and positive likelihood ratio of a PCT level of 0.5 ng/mL or higher were highest for detection of bacterial infection, although the sensitivity was low.
The authors noted that RA is a chronic inflammatory disease accompanied by many complications and that serious infections are associated with many of the advanced therapeutics used to manage it.