Method for the selective measurement of amino-terminal variants of procalcitonin.

Background: Procalcitonin (PCT) is an established marker for diagnosing and monitoring bacterial infections. Full-length PCT [116 amino acids that make up procalcitonin (PCT1-116)] can be truncated, leading to des-Ala-Pro-PCT (des-Alanin-Prolin-Procalcitonin; PCT3-116). Current immunoassays for PCT ("total PCT") use antibodies directed against internal epitopes and are unable to distinguish amino-terminal PCT variants. Here we describe the development of monoclonal antibodies recognizing the amino-termini of PCT1-116 and PCT3-116 and their use in the selective measurement of these PCT species.

Methods: With newly developed monoclonal antibodies against the amino-termini of PCT1-116 and PCT3-116, and an antibody against the katacalcin moiety of PCT, we developed and characterized immunoluminometric assays for the 2 PCT peptides. We comparatively assessed the kinetics of PCT variants in a human endotoxemia model.

Results: Monoclonal antibodies against the amino-termini of PCT1-116 and PCT3-116 showed <1% cross-reactivity with other PCT-related peptides. The sandwich assays for PCT1-116 and PCT3-116 had functional assay sensitivities of 5 and 1.2 pmol/L, respectively, and exhibited recoveries within 20% of expected values. Plasma PCT1-116 was stable for 6 h at 22 degrees C and 24 h at 4 degrees C, and PCT3-116 was stable for at least 24 h at both temperatures. During experimental endotoxemia in healthy people, both PCT1-116 and PCT3-116 increased early in parallel with total PCT, but further increases in PCT1-116 were significantly slower than for PCT3-116 (P = 0.0049) and total PCT (P = 0.0024).

Conclusions: The new assays selectively measure PCT1-116 and PCT3-116. Both PCT species increase early during endotoxemia but differ in their kinetics thereafter. The selective measurement of PCT species with different in vivo kinetics may be useful in improving PCT-guided therapies.