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: Higher optical density of an antigen assay predicts thrombosis in patients with heparin - induced thrombocytopenia .
### Output Text:
Higher optical density of an antigen assay predicts thrombosis in patients with <span class="chemical">heparin</span> - induced thrombocytopenia . 