Validation of a multiwell γ-counter for measuring high-pressure liquid chromatography metabolite profiles

Objectives: The purpose of this study was to verify the accuracy and reproducibility of a multiwell counter to assess its suitability for use within human PET studies in which metabolizing ¹¹C tracers are used. Such tracers often require metabolite analysis for deriving plasma metabolite-corrected input curves. High-pressure liquid chromatography (HPLC) with on-line activity measurement is often unreliable for later plasma samples due to the poor sensitivity of the on-line activity detector. Fraction collector obtained HPLC samples that are counted in a separate high-sensitivity well counter can be an alternative to overcome poor counting statistics. Methods: Several experiments to evaluate background counting, reproducibility, and linearity were performed to validate the accuracy, precision, and detection limits of the well counter. In addition, measurements on a series of samples resembling activity profiles as seen within human ¹¹C-flumazenil studies were performed to evaluate the performance of the well counter for clinically relevant data. Results: The tests proved that the well counter detection limit, linearity, and reproducibility were more than sufficient in circumstances as seen during patient studies for samples with both high and low activity. Conclusion: The use of a multiwell counter is a good alternative for the on-line activity detector of the HPLC, allowing derivation of plasma metabolite fractions with high accuracy and reproducibility.