In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: This observation suggests that increased susceptibility to Warfarin - induced artery calcification could be related to higher serum phosphate levels .
### Output Text:
This observation suggests that increased susceptibility to <span class="chemical">Warfarin</span> - induced artery calcification could be related to higher serum <span class="chemical">phosphate</span> levels . 