For many citation styles, DOIs (digital object identifiers) are used. The following resources will help in working with DOIs.
This style guide handout contains general APA guidance and examples of common formats. The document is one page, double sided.
The handout should not be used as a substitute for consulting with the official style guide manuals.
Below is a sample citation from the APA Style Experts on how to cite a Clinical Trial from clinicaltrials.gov:
Patient Centered Cloud-based Electronic System: Ambient Warning and Response Evaluation (ProCCESs AWARE). (2014). Retrieved from http://clinicaltrials.gov/ct2 (Identification No. NCT02039297)
(It should be double-spaced and use a hanging indent, formatting not available in this box.)