Liz,
There are a lot of very convincing studies to support the use and accuracy of PET scans.

All scans have their limitations and PET scans can not detect cancer clusters that are very small (depending on the scanner, less than ~.5 to ~1cm in size). Also slowly growing, well-differentiated, or small lesions exhibit little or no SUV uptake.

I also understand that the patients glucose levels at the time of the scan is a factor

I believe most PET scans these days are PET/CT scans and despite the limitation, this is considered to be the best imaging tool for the detection of cancer tumors.