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: Compared with control patients , CRF and ESRD patients had higher preoperative serum creatinine levels , a greater percentage of patients with hepatorenal syndrome , higher percentage requirement for dialysis in the first 3 months postoperatively , and a higher 1 - year serum creatinine .
### Output Text:
Compared with control patients , CRF and ESRD patients had higher preoperative serum <span class="chemical">creatinine</span> levels , a greater percentage of patients with hepatorenal syndrome , higher percentage requirement for dialysis in the first 3 months postoperatively , and a higher 1 - year serum <span class="chemical">creatinine</span> . 