Serum procalcitonin in systemic autoimmune diseases--where are we now?

To review the current evidence regarding the value of measuring procalcitonin (PCT) levels in patients with systemic autoimmune diseases, with a focus on the evidence for diagnostic and analytical performance of this biomarker. A brief description of the pathophysiological basis of this biomarker is also included.

Using PubMed from the National Library of Medicine, relevant English literature on PCT in patients with different systemic autoimmune diseases, from 1990 to 2009, was reviewed. The search used keywords referring to procalcitonin and systemic lupus erythematosus, antineutrophil cytoplasmic antibody-associated systemic vasculitis, Goodpasture syndrome, rheumatoid arthritis, and giant cell arteritis.

When used in the appropriate clinical setting, the measurement of serum PCT levels is valuable as a marker of severe systemic bacterial and fungal infections and sepsis. Information regarding plasma PCT levels in patients with active underlying systemic autoimmune diseases is limited, primarily from observational studies and case series, with considerable variability of patient characteristics and clinical settings. In the detection of systemic infection concomitant with autoimmune diseases, PCT had a diagnostic sensitivity of 53 to 100% and a specificity of 84 to 97% (depending on the selection criteria) and was superior to other inflammatory markers tested. Most of the studies used a semiquantitative test for PCT measurement (functional assay sensitivity <0.5 ng/mL), which can explain the low sensitivity of the test. PCT levels were not significantly affected by renal function abnormalities or immunosuppressive agents. Although high PCT levels commonly occurred with infection, elevated levels of PCT could be found in patients with vasculitis without evidence of infection, often correlated with high disease activity scores.

Significantly elevated PCT levels offer good specificity and sensitivity for systemic infection in patients with systemic autoimmune diseases, regardless of the use of corticosteroids or immunosuppressive agents. PCT measurement may add to diagnostic accuracy in patients with systemic autoimmune diseases who present with a febrile illness, especially when highly sensitive PCT assays and specific PCT cutoff ranges are used in a predefined clinical setting (reflecting the likelihood of infection versus an autoimmune disease flare). However, there are limitations when using this biomarker in patients with systemic autoimmune diseases. PCT levels should not replace the necessary extensive diagnostic workup, which should include a thorough history and physical examination, combined with appropriate immunological, microbiological, radiological, and histological data.