Can I dispute this denied health insurance claim?

Yes, there is always a way to file a complaint.

See if they go by date of service or when the bill was actually put thru. If you had insurance up till the 15th than u are covered. The person doing your claim may have made a mistake.

Yeah, they have to pay - they just don't want to, and hope you wont bother to make them. Veronica 91 is talking about COBRA which can be pricey but probably less so than a medical bankruptcy; I know because I have it for my daughter who turned 27 in Decemeber.

Insurance companies can be a bunch of big, dumb turkeys. I worked for one for 15 years

Insurance companies make mistakes all the time. Call them.

You can of course dispute the bill. Have you taken advantage of continuing on your employers insurance for as many months as is allowed? Can't remember what it is called right now. You have a certain length of time to elect this and it may be as short as 2 months which may mean you have to do something today. if you already have insurance from a new employer that won't help you

Is this employer based insurance or personally owned coverage? If Employer, call the HR department and seek assistance from them. If you owned the coverage on your own, you will have to call the claims help number and see what the issue is. And yes, all insurance claims can be appealed. BUT if coverae ended before the date of service, it may not change anything.