Part one of this memorandum provides a summary of questions asked and comments submitted in response to the DOL request for information ("RFI") about the definition of "Serious health condition".
Part two of this memorandum contains the relevant statutory and regulatory text. Part two also lists other sources cited in the comments about this topic.
Workplace Flexibility 2010, Georgetown University Law Center, "The Definition of “Serious Health Condition”" (2007). Memos and Fact Sheets. Paper 34.