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 these patients , the magnitude of proteinuria was assessed on morning urine samples by turbidometric measurement or random urine protein : creatinine ratios , an estimate of grams of proteinuria / day .
### Output Text:
In these patients , the magnitude of proteinuria was assessed on morning urine samples by turbidometric measurement or random urine protein : <span class="chemical">creatinine</span> ratios , an estimate of grams of proteinuria / day . 