White Blood Cells Can Predict Steroid Response in OA

Brief Report: Synovial Fluid White Blood Cell Count in Knee Osteoarthritis: Association With Structural Findings and Treatment Response

McCabe PS, Parkes MJ, Maricar N, et al
Arthritis Rheumatol. 2017;69:103-107

Summary

In this open-label study, 55 individuals with knee osteoarthritis (OA) had synovial fluid sampled before intra-articular injection of methylprednisolone. The white blood cell (WBC) count in synovial fluid was assessed and divided into three categories: ≤ 100 cells/µL, 101-250 cells/µL, and 251-1000 cells/µL. Of note, patients with a WBC count > 1500 cells/µL were removed from the study because of concerns that they may have primary inflammatory arthritis.

Questionnaires and clinical data were collected both pre- and postinjection, with the latter typically occurring within 2 weeks of the injection. In a subset of patients, pre- and postinjection MRI of the knee was also obtained.

Before injection, the authors found no significant associations between synovial fluid WBC categories and age, pain, or OA disease severity. However, individuals in the highest category of synovial fluid WBC count had the most improvement in pain, as measured by the Knee Injury and Osteoarthritis Outcome Score. In the subset of patients who had MRI, the highest category of WBC count was associated with greater observed synovitis, cartilage loss, and bone marrow lesions, as well as a trend toward greater within-person improvement of synovitis after injection.

Viewpoint

This small but intriguing study provides some evidence that higher synovial fluid WBC counts—even within a "normal" range—can be a fair biomarker to predict response to intra-articular steroids. This will need further exploration, but if these findings hold up in robust larger studies, they will be a welcome addition to clinical care where synovial fluid WBC counts are readily obtainable. Furthermore, these findings fit nicely with the growing understanding that some aspects of OA are inflammatory,^[1] and will hopefully contribute to additional understanding of the pathophysiology of disease and mechanism-based therapy in the future.