Serum Test Detects Bacterial Infection in Autoimmune Diseases

Scientists from Chang Gung University (Tao Yuan, Taiwan) have reviewed studies carried out between January 1966 and October 2011 that had evaluated PCT as a diagnostic marker for bacterial infection in patients with AD, which included nine studies for PCT and five studies for C-reactive protein (CRP).

In healthy individuals, serum PCT is normally undetectable below 0.05 ng/mL, but the level increases rapidly after bacterial infection. In contrast to CRP, PCT does not rise with noninfectious inflammation or nonbacterial infections, making it a potentially useful marker to distinguish bacterial infection from disease flare in the setting of autoimmune disease.

The positive likelihood ratio for PCT was sufficiently high to be qualified as a rule-in diagnostic tool, while the negative likelihood ratio was not sufficiently low to be qualified as a reliable rule-out diagnostic tool. The area under the receiver-operating curve was higher for PCT than for CRP. Both markers had suboptimal negative likelihood ratios and so are not suitable tests for excluding bacterial infection in febrile patients with autoimmune diseases.

The authors conclude that different cut off levels are needed to optimize the discriminative capability of PCT for different autoimmune diseases. However, this requires a large study or studies of patients with the same autoimmune disease.

Until the results of such studies are available, they recommend using a standard PCT cut off value of 0.5 ng/mL, which has reasonable sensitivity of 76% and specificity of 88%.

Shy-Shin Chang, MD, lead author of the study said, "Analysis of the pooled data suggests that PCT is a more specific indicator of bacterial infection than CRP, but that CRP is a more sensitive indicator of bacterial infection than PCT." The study was published online on May 17, 2012, in the journal Arthritis and Rheumatism.

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