Serum and synovial fluid analysis for diagnosing chronic periprosthetic infection in patients with inflammatory arthritis.

BACKGROUND The serum erythrocyte sedimentation rate and C-reactive protein level, as well as the synovial fluid white blood-cell count with differential, are commonly used tests for the diagnosis of periprosthetic joint infection; however, their utility for the diagnosis of periprosthetic joint infection in patients with inflammatory arthritis is unknown. METHODS Eight hundred and three patients undergoing 871 consecutive hip and knee arthroplasties (including sixty-one in patients with inflammatory arthritis and 810 in patients with noninflammatory arthritis) were prospectively evaluated for periprosthetic joint infection. The erythrocyte sedimentation rate, C-reactive protein level, and synovial fluid white blood-cell count with differential were obtained routinely. Receiver operating characteristic curves were used to establish optimal thresholds for the diagnosis of periprosthetic joint infection, and the area under the curve was calculated to determine the overall accuracy of these tests for patients with inflammatory compared with noninflammatory arthritis. RESULTS The utility of all serum and synovial tests for predicting chronic periprosthetic joint infection was similar for patients with noninflammatory and inflammatory arthritis. The optimal cutoffs in patients with noninflammatory and inflammatory arthritis were 32 and 30 mm/hr, respectively, for the erythrocyte sedimentation rate; 15 and 17 mg/L, respectively, for the C-reactive protein level; 3450/μL and 3444/μL, respectively, for the synovial fluid white blood-cell count; and 78% and 75%, respectively, for the differential. The areas under the curves were similar for the two groups (84.9% and 85.0%, respectively, for the erythrocyte sedimentation rate; 88.5% and 85.1%, respectively, for the C-reactive protein level; 94.5% and 93.8%, respectively, for the synovial fluid white blood-cell count, and 95.0% and 93.6%, respectively, for the differential). Finally, the sensitivities, specificities, negative predictive values, and positive predictive values for all tests were also comparable in both groups. The rate of periprosthetic joint infection was significantly higher following procedures in patients with inflammatory arthritis than following procedures in patients with noninflammatory arthritis (31% compared with 18%; p = 0.013). CONCLUSIONS The erythrocyte sedimentation rate, C-reactive protein level, and synovial fluid white blood-cell count with differential are useful for diagnosing periprosthetic joint infection in patients with inflammatory as well as noninflammatory arthritis, with similar optimal cutoff values and overall testing performance. The synovial fluid white blood-cell count and differential performed the best for the diagnosis of periprosthetic joint infection. Physicians evaluating patients with a failed or painful total hip or knee arthroplasty should not assume that elevation of the erythrocyte sedimentation rate, C-reactive protein level, and synovial fluid white blood-cell count with differential is secondary to inflammatory arthropathy; rather, elevation of these markers may indicate periprosthetic joint infection, and further evaluation for infection is warranted.