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: PATIENTS AND METHODS : Patients with more than 50 % decrease in platelet count or thrombocytopenia ( < 150 x 10 ( 9 ) / L ) after exposure to heparin , who had a positive two - step antigen assay [ optical density ( OD ) > 0 . 4 and > 50 inhibition with high concentration of heparin ] were included in the study .
### Output Text:
PATIENTS AND METHODS : Patients with more than 50 % decrease in platelet count or thrombocytopenia ( < 150 x 10 ( 9 ) / L ) after exposure to <span class="chemical">heparin</span> , who had a positive two - step antigen assay [ optical density ( OD ) > 0 . 4 and > 50 inhibition with high concentration of <span class="chemical">heparin</span> ] were included in the study . 