In the sentence extracted from biomedical literature, identify all the disease entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="disease"> to denote a disease.

### Input Text: Using a specific radioimmunoassay , we determined colchicine concentrations in the serum and breast milk of 4 patients at various time points , following oral administration of the drug .
### Output Text:
Using a specific radioimmunoassay , we determined colchicine concentrations in the serum and breast milk of 4 patients at various time points , following oral administration of the drug . 