The cut off points of 2.5-3.5 means that:
if your SUVmax>3.5 you MAY have tumor activity within that tumor.(positive)
if your SUVmax is 2.5-3.5 ,you may or may not have any meaningful tumor activity even the "hot spots" light up on the PET scan. (stable)
if your SUVmax<2.5, the possibility of having active tumor is less. (negative)
However, the above values and judgement by the radiologist depends on comparing the overall uptake of the radioactive glucose of the tumor in the liver to the blood pool around the lungs, or other point of reference chosed by him/her for that single PET scan.
The SUVmax value also depending on your height weight at time of the scan as well as the dose given by the radiology technician and the waiting time (usually 60 min.) between the injection and the start of the scan; the breathing (movement of the chest)of the patient also may be one of the factor that can affect the value.
BTW, inflammation caused by infection will also cause false-positive results of the SUVmax value.
But the most usefullness of PET scan is to compare YOUR most current one to the one YOU had taken before(ie: every 3 or 6 month etc.).That is in that series of comparing scans that will provide you the best answer of your disease progress.
so even you had a SUVmax value of 4.6 from your July 27 message . that value should be compared with your most current PET to see whether your chemotherapy is working or not.
According to the radiologist I talked to,the SUVmax value is relative and not absolute .
I hope the above can help a little in understanding about PET scan.
Please know that my personal opinion is not intended nor implied to be a substitute for professional medical advice. If provided, information are for educational purposes.Consult doctor is a MUST for changing of treatment plans.