Serum procalcitonin for differentiating bacterial infection from disease flares in patients with autoimmune diseases

Abstract

Early differentiation between bacterial infections and disease flares in autoimmune disease patients is important due to different treatments. Seventy-nine autoimmune disease patients with symptoms suggestive of infections or disease flares were collected by retrospective chart review. The patients were later classified into two groups, disease flare and infection. C-reactive protein (CRP) and serum procalcitonin (PCT) levels were measured. The CRP and PCT levels were higher in the infection group than the disease flare group (CRP,11.96 mg/dL ± 9.60 vs 6.42 mg/dL ± 7.01, P = 0.003; PCT, 2.44 ng/mL ± 6.55 vs 0.09 ng/mL ± 0.09, P < 0.001). The area under the ROC curve (AUC; 95% confidence interval) for CRP and PCT was 0.70 (0.58-0.82) and 0.84 (0.75-0.93), which showed a significant difference (P < 0.05). The predicted AUC for the CRP and PCT levels combined was 0.83, which was not significantly different compared to the PCT level alone (P = 0.80). The best cut-off value for CRP was 7.18 mg/dL, with a sensitivity of 71.9% and a specificity of 68.1%. The best cut-off value for PCT was 0.09 ng/mL, with a sensitivity of 81.3% and a specificity of 78.7%. The PCT level had better sensitivity and specificity compared to the CRP level in distinguishing between bacterial infections and disease flares in autoimmune disease patients. The CRP level has no additive value when combined with the PCT level when differentiating bacterial infections from disease flares.

Keywords: Autoimmune Diseases; Bacterial Infections; C-reactive Protein; Procalcitonin.

Fig. 1

Box-plot diagram for CRP in disease flare and infection groups, with box encompassing the range of levels from the 25th percentile (lower bar) to the 75th percentile (upper bar). The horizontal line within the box indicates the median value. The extreme values (within 1.5 times the inter-quartile range [IQR] from the upper or lower quartile) are the ends of the lines extending from the IQR.

Fig. 2

Boxplot for procalcitonin (PCT) in disease flare and infection groups. The explanation is the same as in Fig. 1.

Fig. 3

Receiver operating characteristics (ROC) curve for CRP and PCT levels in differentiating bacterial infections from disease flares. The area under the curve (AUC) was 0.70 (95% CI, 0.58-0.82) for CRP and 0.84 (95% CI, 0.75-0.93) for PCT. Predicted AUC for CRP and PCT combined was 0.83.