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: The hemoglobin concentration in patients with > or = 2 g / dL decrease at week 2 was observed to be significantly lower even after week 2 than in patients with < 2 g / dL decrease ( P < 0 . 01 ) .
### Output Text:
The hemoglobin concentration in patients with > or = 2 g / dL decrease at week 2 was observed to be significantly lower even after week 2 than in patients with < 2 g / dL decrease ( P < 0 . 01 ) . 