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: Patients were divided into three groups : Controls , no CRF or ESRD , n = 748 ; CRF , sustained serum creatinine > 2 . 5 mg / dl , n = 41 ; and ESRD , n = 45 .
### Output Text:
Patients were divided into three groups : Controls , no CRF or ESRD , n = 748 ; CRF , sustained serum <span class="chemical">creatinine</span> > 2 . 5 mg / dl , n = 41 ; and ESRD , n = 45 . 