Pre-analytical and analytical validation of calcitonin measurement in fine needle aspiration cytology washout.

OBJECTIVES: Current guidelines advise calcitonin measurements in washout from fine needle aspiration cytology (FNAC) upon inconclusive cytological examination of medullary thyroid cancer (MTC). (Pre-)analytical validation studies of calcitonin measurements in FNAC washout are however missing. In this study, we examined (pre-)analytical aspects of calcitonin measurements using three washout solvents, aiming to provide insights for implementation of calcitonin measurements in FNAC washout as a supplementary test for diagnosing and monitoring patients with MTC. METHODS: Validation studies were performed using CytoLyt, NaCl and MultiDiluent as washout solvents. Precision (intra- and inter-assay coefficients of variation (CV)), accuracy (linearity and recovery) and storage stability of each solvent were assessed after spiking with calcitonin standard. Calcitonin measurements were performed using the serum CALCT immunoassay (Siemens, Atellica). Decision criteria were set based on CLSI and manufacturer's manual. RESULTS: Measurements of calcitonin in MultiDiluent met all decision criteria: intra- and inter-assay CVs were ≤5.5 % and ≤9 %, serial dilutions were linear (R2=0.997) and median recovery was 148 % with limited variation. Furthermore, calcitonin concentrations remained stable (<10 % change) after storage. Measurements in CytoLyt were precise and accurate, but unstable upon storage. Measurements in NaCl only met the criteria for intra-assay CV and linearity. CONCLUSIONS: This study showed that MultiDiluent was the optimal solvent to use for calcitonin measurements in a non-serum matrix using the Atellica calcitonin immunoassay. CytoLyt performed suboptimal, while the performance of NaCl was poor. Further research is needed to validate the recommended work-up using patient-derived material.