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 both groups postprocedural ( 48 h ) serum creatinine was measured .
### Output Text:
In both groups postprocedural ( 48 h ) serum <span class="chemical">creatinine</span> was measured . 