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: Multivariate stepwise logistic regression analysis using preoperative and postoperative variables identified that an increase of serum creatinine compared with average at 1 year , 3 months , and 4 weeks postoperatively were independent risk factors for the development of CRF or ESRD with odds ratios of 2 . 6 , 2 . 2 , and 1 . 6 , respectively .
### Output Text:
Multivariate stepwise logistic regression analysis using preoperative and postoperative variables identified that an increase of serum <span class="chemical">creatinine</span> compared with average at 1 year , 3 months , and 4 weeks postoperatively were independent risk factors for the development of CRF or ESRD with odds ratios of 2 . 6 , 2 . 2 , and 1 . 6 , respectively . 