[en] In the present work an ultra-high-pressure liquid chromatography–tandem mass spectrometry (UHPLC–MS/MS) method for the simultaneous determination of cholecalciferol and 24,25(OH)2D3 in serum has been developed and validated. The validation was carried out by analyzing serum fortified at five concentration levels. In all cases, recoveries ranging from 89-109% for cholecalciferol and 85-111% for 24,25(OH)2D3 and CV below 7% were attained with this approach. As a consequence, this procedure is ready to assess the rates of synthesis of cholecalciferol and 24,25(OH)2D3 in the skin either by sun exposition or by oral absorption.
