A diagnostic device comprising an antibody, or an antigen-binding fragment thereof, that binds 25-hydroxyvitamin D2 and 25-hydroxyvitamin D3 in a sample, wherein the antibody comprises a heavy chain and a light chain, wherein the heavy chain comprises a heavy chain complementarity determining region (CDR) 1 comprising the amino acid sequence of SEQ ID NO:10, a heavy chain CDR2 comprising the amino acid sequence of SEQ ID NO:11, and a heavy chain CDR3 comprising the amino acid sequence of SEQ ID NO:
- 12, and wherein the light chain comprises a light chain CDR1 comprising the amino acid sequence of SEQ ID NO:26;
- a light chain CDR2 comprising the amino acid sequence of SEQ ID NO:
- 27;
- and a light chain CDR3 comprising the amino acid sequence of SEQ ID NO:28.