Adaptation of a routine immunoassay analyser for TSH measurement in dried blood spot and volumetric microsampling (Capitainer B50): evaluation for thyroid function testing in adults

Background Thyroid-stimulating hormone (TSH) is the primary biomarker for diagnosing and monitoring thyroid disorders. Although serum remains the reference matrix, microsampling approaches, including dried blood spots (DBS) and volumetric devices (CPT B50), may improve accessibility and compliance. This study evaluated the analytical and clinical performance of TSH measurement from DBS and CPT B50 compared with serum. Methods Paired serum, DBS, and CPT B50 samples were collected from 246 adults undergoing thyroid evaluation. DBS and CPT B50 samples were eluted and analyzed using a chemiluminescence immunoassay (RUO, LIAISON® XL, DiaSorin). Analytical performance, recovery, repeatability, stability, and method agreement were assessed. Microsample results were converted to serum-equivalent concentrations using inverse Passing–Bablok regression equations. Performance was evaluated using Passing–Bablok regression and Bland–Altman analysis with predefined acceptance criteria (bias ≤ ±15%, CV ≤ 15%, recovery 85%–115%), guided by CLSI EP15-A3 and EP17-A2 guidelines. Results Repeatability CVs ranged from 3.7% to 10.4%, and recovery was 97%–110%. DBS and CPT B50 showed strong agreement with serum, with mean bias of −0.10 mIU/L for CPT B50 and 0.24 mIU/L for DBS. Samples remained stable for up to 8 weeks at +4 °C and −20 °C. Thyroid status classification accuracy was 96% for DBS and 98% for CPT B50. Conclusions TSH measurement from DBS and CPT B50 showed high analytical and clinical concordance with serum. Volumetric sampling improved reproducibility and stability, supporting its use in remote thyroid testing and therapy monitoring.