To Convert or Not to Convert? Official International Association of Therapeutic Drug Monitoring and Clinical Toxicology Guideline: Considerations and Recommendations for Converting Capillary Blood Microsampling Concentrations to Plasma Concentrations

Capillary blood microsampling enables the sampling of small blood volumes, making it suitable for vulnerable populations, remote collection, and repeated sampling. Owing to its minimal invasiveness, blood microsampling has emerged as an alternative to venipuncture in many fields. However, as capillary blood differs from venous blood in composition and analytes vary in their distribution between the blood cell and plasma fractions, plasma and capillary blood concentrations may differ. Because plasma is often the standard matrix for routine analyses, this discrepancy can compromise the clinicians' interpretation of capillary blood results. Hence, to ensure comparability with results obtained through plasma-based assays and reference ranges, accurate conversion to plasma concentrations may be required, which may improve the reliability and validity of microsampling-based outcomes. Despite its relevance, guidance on whether conversion is appropriate or desired and on how to translate and validate the translation of capillary blood microsampling results to plasma concentrations is lacking.