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: Serum creatinine ( SCr ) levels and estimated glomerular filtration rate were assessed at baseline and 2 to 5 days after receiving medications .
### Output Text:
Serum <span class="chemical">creatinine</span> ( SCr ) levels and estimated glomerular filtration rate were assessed at baseline and 2 to 5 days after receiving medications . 