ABSTRACT Background Calcitonin doubling time (CT‐DT) is an established prognostic tool in medullary thyroid carcinoma (MTC), yet CT measurement can be affected by analytical variability and assay interference. Procalcitonin (PCT) has shown promise as a complementary biomarker, but data on PCT doubling time (PCT‐DT) remain scarce. Methods Longitudinal CT and PCT were measured using Elecsys immunoassays in 25 MTC patients followed for up to 24 months after thyroidectomy. Doubling times were estimated using log‐linear regression and a rolling‐window approach. Agreement was assessed using Spearman correlation, Bland–Altman analysis, and Cohen's κ across predefined DT categories. Results CT and PCT kinetics showed clear separation across response categories. In progressive disease, CT‐DT and PCT‐DT were strongly correlated with minimal systematic bias and substantial categorical agreement ( κ = 0.76). Short DT values clustered in structural incomplete responders. Conclusions PCT‐DT closely mirrors CT‐DT and may serve as a complementary kinetic marker in the postoperative monitoring of MTC, particularly when CT interpretation is uncertain; however, these findings should be considered exploratory and warrant confirmation in larger prospective cohorts before clinical implementation.
Campenni et al. (Tue,) studied this question.