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: The cumulative incidence of serum creatinine levels > 50 % above normal was 33 % after 96 weeks .
### Output Text:
The cumulative incidence of serum <span class="chemical">creatinine</span> levels > 50 % above normal was 33 % after 96 weeks . 