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: Increased serum creatinine was observed in 73 % of patients in OD versus 57 % of patients in TD , without evidence of nephrotoxicity .
### Output Text:
Increased serum <span class="chemical">creatinine</span> was observed in 73 % of patients in OD versus 57 % of patients in TD , without evidence of nephrotoxicity . 