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: Mean serum creatinine level before conversion was 2 . 21 mg / dL and thereafter , 4 . 93 mg / dL ( P = . 02 ) .
### Output Text:
Mean serum <span class="chemical">creatinine</span> level before conversion was 2 . 21 mg / dL and thereafter , 4 . 93 mg / dL ( P = . 02 ) . 