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: Children with persistent sterile leukocyturia more frequently had serum creatinine levels of 50 % above normal than those children without persistent sterile leukocyturia .
### Output Text:
Children with persistent sterile leukocyturia more frequently had serum <span class="chemical">creatinine</span> levels of 50 % above normal than those children without persistent sterile leukocyturia . 