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: MEASUREMENTS : Platelet counts , onset of objectively determined thromboembolism , results of heparin - induced platelet factor 4 antibody tests , and outcomes .
### Output Text:
MEASUREMENTS : Platelet counts , onset of objectively determined thromboembolism , results of <span class="chemical">heparin</span> - induced platelet factor 4 antibody tests , and outcomes . 