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 : Median maximum serum levetiracetam concentration was 25 . 5 microg / mL , median minimum serum levetiracetam concentration was 8 . 3 microg / mL , and median elimination half - life was 2 . 9 hours .
### Output Text:
RESULTS : Median maximum serum <span class="chemical">levetiracetam</span> concentration was 25 . 5 microg / mL , median minimum serum <span class="chemical">levetiracetam</span> concentration was 8 . 3 microg / mL , and median elimination half - life was 2 . 9 hours . 