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: Serum sodium ( Na ) , potassium ( K ) , and creatinine ( Cr ) were measured 24 h before angiography as baseline values , then measured again at 12 - , 24 - , and 48 - h intervals after CM use .
### Output Text:
Serum <span class="chemical">sodium</span> ( <span class="chemical">Na</span> ) , <span class="chemical">potassium</span> ( <span class="chemical">K</span> ) , and <span class="chemical">creatinine</span> ( <span class="chemical">Cr</span> ) were measured 24 h before angiography as baseline values , then measured again at 12 - , 24 - , and 48 - h intervals after <span class="chemical">CM</span> use . 