I am trying to figure out if CPAP machines count as a personal item, or if it is like other US airlines whereby the CPAP is a medical device, and you can still have a personal item with you in the cabin. I have checked the Turkish airlines website and their FAQs, and it is not clear to me what the answer is. They say that mobility devices would not be classified as a personal item, but they don't have any information about CPAP machines. They have an FAQ about respiratory devices, etc., but still no answer about whether or not I can take my CPAP in addition to a personal device in the cabin. I hate to have to call them, but I did see something about an RS feed (in other topic answers) and wonder what that is, maybe I can go that route if no one here knows the answer (if I can figure out what that is exactly). Thank you for any information or advice.
Did you try sending a private message to the person that posted in the other topic to see what happened with them? Have you checked more specialized websites like Flyertalk? If no answers there then it is probably a question best answered by the airline.
I've been traveling with a CPAP machine for a number of years, and have never had an airline refuse to allow me to bring it on as an extra personal item. That said, I have not flown on Turkish Airlines, but I would be willing to bet good money that you should not have any problems bringing it on.
EDIT: I just did a search and found this info from the TA website: https://www.turkishairlines.com/en-int/any-questions/special-assistance-for-passengers/
In case you are traveling with personal respirator/ POC/CPAP machine please make sure that the device is FAA approved. You may use your own devices whenever you need to, except during take-off and landing. Passengers traveling with portable respirators must inform the reservation unit 48 hours before the flight time regarding the transportation and/or use of the devices. A doctor’s certificate is required if the passenger intends to use the portable respirator in the cabin. Please keep in mind that if you intend to use your inhaler on board, you should bring enough batteries (enough to last one and a half times the flight duration). There is no requirement to have a power outlet in the cabin for these devices.
So it sounds like you can bring it on with no problem.