A Certified Spiking Solution® of 3-epi-25-Hydroxyvitamin D3, the epi isomer of the prohormone 25-hydroxyvitamin D3. The presence of 3-epi-25-hydroxyvitamin D3 can affect accurate quantitation of 25-hydroxyvitamin D3 levels by most routine LC-MS/MS methods and to an unknown extent in immuno- and protein binding assays. This solution standard is suitable for critical quantitative applications in research and clinical environments including: for use in preparation of controls and calibrators for LC/UV and LC-MS/MS assays; as system tuning/suitability standards; linearity verification; internal proficiency testing; and for OEM use in preparation of reagents, development of assays and instrument IQ/OQ.