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: In the patients , but not the healthy subjects , these symptoms were significantly correlated with plasma caffeine levels .
### Output Text:
In the patients , but not the healthy subjects , these symptoms were significantly correlated with plasma <span class="chemical">caffeine</span> levels . 