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: Results of serologic tests for heparin - induced antibodies were positive in all patients .
### Output Text:
Results of serologic tests for <span class="chemical">heparin</span> - induced antibodies were positive in all patients . 