Cardiology qtc question based on 24 hr holter report: if max qt is 548 and max qtc is 586, is that a red flag? could cold fx cause that?

Abnormal: Prolonged qt can be associated with serious arrhythmias . Medicines can cause this. There are also genetic causes. See your doctor or cardiologist asap.

Depends: it depends upon the quality of the data, certainly the telemetry measurement suggested by the holter is quite concerning. the gold standard would be the longest QTc interval on a good quality ECG. If indeed the QTc is that long, you should see a cardiologist.