Laboratory Markers in Osteoarthritis
In osteoarthritis, inflammatory markers including CRP and ESR are typically normal or only mildly elevated, and blood tests are not required for diagnosis. 1
Key Laboratory Findings
Inflammatory Markers Are Usually Normal
ESR, rheumatoid factor (RF), and CRP are usually normal/negative or only mildly elevated/positive in non-erosive osteoarthritis, distinguishing it from inflammatory arthritides like rheumatoid arthritis. 1
Blood tests are not required for the diagnosis of osteoarthritis but may be needed to exclude coexistent inflammatory disease. 1
More pronounced abnormalities in inflammatory markers should prompt a search for alternative diagnoses such as rheumatoid arthritis, psoriatic arthritis, or gout. 1
When Inflammatory Markers May Be Elevated
High-sensitivity CRP (hsCRP) can be elevated in some osteoarthritis patients and correlates with local synovial inflammation, with 57% of patients showing inflammatory infiltrates in synovial membranes. 2
Patients with inflammatory infiltrates had significantly higher mean hsCRP levels (4.7 ± 5.0 mg/L) compared to those without inflammation (1.7 ± 3.6 mg/L). 2
Both ESR and hsCRP are significantly higher in knee osteoarthritis patients compared to non-osteoarthritis controls, and these markers correlate with clinical features like tenderness, swelling, and patellar ballottement. 3
In knee osteoarthritis, hsCRP concentration may increase even in early-stage disease (Kellgren-Lawrence grade II). 3
Clinical Interpretation
When a patient with osteoarthritis has marked inflammatory symptoms and/or signs, especially involving atypical sites, blood tests should be undertaken to screen for additional inflammatory arthritides. 1
The correlation between ESR/CRP and disease activity measures in osteoarthritis is weak, suggesting these markers have limited utility for monitoring disease progression. 4
ESR and CRP are modestly correlated with each other but poorly correlated with clinical measures of disease activity in osteoarthritis patients. 4
Important Clinical Pitfalls
Do not rely on normal inflammatory markers to exclude inflammatory arthritis in a patient with atypical osteoarthritis presentations—consider other clinical and investigational features such as marginal erosions, elevated serum uric acid, or crystal identification. 1
Approximately 28% of results show discordance between ESR and CRP, with CRP being a better measure of acute phase response when discordance occurs. 5
ESR is influenced by non-acute phase factors including immunoglobulins, rheumatoid factor, and hemoglobin levels, which can lead to misleading elevations unrelated to inflammation. 5