Several databases, including CINAHL, do not include the DOI number. You will need to go to the fulltext article to find the DOI number. In most cases, the DOI is included with the Journal Name at either the top or bottom of the first page of the article.
Alternatively, you can look up the DOI number if you have the citation information.
1. Go to CrossRef
2. Choose Simple Text Query.
3. Copy and paste your citation information and CrossRef will supply the DOI number.