On one hand in the Qur'an we have a clear statement saying:
[Fasting for] a limited number of days. So whoever among you is ill or on a journey [during them] - then an equal number of days [are to be made up]. And upon those who are able [to fast, but with hardship] - a ransom [as substitute] of feeding a poor person [each day]. And whoever volunteers excess - it is better for him. But to fast is best for you, if you only knew. (2:184)
this means one is allowed not to fast if one is sick.
Nevertheless scholars say if one is able to fast it is better to do so. Others say that in case of doubt one should refer to a specialist -a physician- who may "give a verdict" whether fasting would be harmful or not, and most of those also say one should ask a good -preferably practicing- Muslim doctor and not rely on a non-Muslim physician. Some evidences for referring to a specialist usually are:
... So ask the people of the message if you do not know.(16:43)
... so ask about Him one well informed.(25:59)
... Make use of medical treatment, for Allah has not made a disease without appointing a remedy for it, with the exception of one disease, namely old age. (See for example in Sunan abi Dawod)
In my experience most Muslim physicians are rather more cautious or afraid to give a verdict or to ask a person to refrain from fasting Ramadan as some scholars would be. In best case they would ask you to try fasting, this becomes very difficult once a person is in need to take a medicine at specific times or for example needs to drink a lot during the day ...
My question is have scholars defined or given any guidelines one could follow to decide when one should refrain from fasting (for example categories determining when a sickness is considered severe, slight, or mild) and when fasting is better even during a time of sickness?