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: However , an increase of serum creatinine at various times postoperatively is more predictive of the development of CRF or ESRD .
### Output Text:
However , an increase of serum <span class="chemical">creatinine</span> at various times postoperatively is more predictive of the development of CRF or ESRD . 