Diagnostic accuracy of serum procalcitonin concentrations for detecting systemic bacterial infection in patients with systemic autoimmune diseases

Objective. To examine whether serum procalcitonin (PCT) concentrations are useful for distinguishing bacterial infections from disease flares in patients with systemic autoimmune diseases. Methods. Patients with systemic autoimmune diseases who were admitted to our hospitals due to either a suspected deterioration of their primary diseases or an infectious disease were enrolled. Serum PCT levels were measured in 99 serum samples of 98 patients who had elevated serum C-reactive protein (CRP) levels; 29 samples were obtained from patients with bacterial infections, and 70 samples were obtained from patients with disease deterioration without a detectable infection. The diagnostic accuracy, sensitivity, and specificity for identifying a bacterial infection were estimated using the receiver-operating characteristic curve. Multiple logistic regression analysis was also done with PCT level, age, sex, steroid dose, and use of immunosuppressive agents. Results. Serum PCT levels were higher in the bacterial infection group than in the disease flare group (mean ± SD, 4.539 ± 9.677 vs 0.116 ± 0.127; p < 0.0001). The diagnostic accuracy of PCT for bacterial infection was 0.797, sensitivity 53.3%, and specificity 97.1%. On multivariate analysis, the odds ratio of a PCT ≥ 0.5 ng/ml was significant (OR 59.085, 95% CI 7.705-453.088, p < 0.0001) for identifying bacterial infection. Conclusion. Elevated serum PCT levels have a good specificity for diagnosing bacterial infection in patients with systemic autoimmune diseases regardless of their dosage of oral corticosteroids and immunosuppressive agents.