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: After drug administration , creatinine concentrations in the plasma and in the urine of the rats were determined , as well as creatinine clearance .
### Output Text:
After drug administration , <span class="chemical">creatinine</span> concentrations in the plasma and in the urine of the rats were determined , as well as <span class="chemical">creatinine</span> clearance . 