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: A decrease in Hb levels between 2 and 5 g / dL was detected in 23 ( 11 . 4 % ) patients during the follow - up period .
### Output Text:
A decrease in Hb levels between 2 and 5 g / dL was detected in 23 ( 11 . 4 % ) patients during the follow - up period . 