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: In multivariate analysis , three factors independently predicted mortality : serum bilirubin ( > or = 10 . 8 mg / dL ) , prothrombin time ( PT ) prolongation ( > or = 26 seconds ) , and grade III / IV encephalopathy at presentation .
### Output Text:
In multivariate analysis , three factors independently predicted mortality : serum <span class="chemical">bilirubin</span> ( > or = 10 . 8 mg / dL ) , prothrombin time ( PT ) prolongation ( > or = 26 seconds ) , and grade III / IV encephalopathy at presentation . 