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: Subsequently , the serum creatinine levels decreased , the urine albumin / creatinine ratios returned to zero , and the leukocyturia disappeared within 3 months .
### Output Text:
Subsequently , the serum <span class="chemical">creatinine</span> levels decreased , the urine albumin / <span class="chemical">creatinine</span> ratios returned to zero , and the leukocyturia disappeared within 3 months . 