Hi JoHarr, as Darlene is on LTOT (long term oxygen therapy) it's important to remember that a higher than optimal (88-92%) Saturation could indicate some CO2 retention. Bear in mind that depending on the flow of extra oxygen, not everyone is able to ventilate the CO2 and the build up can result in toxicity (Hypercapnia and Respiratory Acidosis).
There is a fine balance to be met when on oxygen therapy, one that is sometimes very difficult to achieve. I couldn't understand why respiratory doctors were reluctant to start my mother on LTOT. I just knew her saturation was dangerously low. But we were never properly informed (but that's another story) about the consequences of Oxygen usage in some COPD patients. Those often referred to as 'retainers'.
I soon realised the consequences when my mother was eventually given supplemental oxygen and she went on to develop CO2 retention. She was unable to tolerate NIV so this left us with very little options when it came to treatment.
97-98% is considered normal for those without lung disease, but in those with poor lung function it may indicate a poor gas exchange. I'm in no way an expert on these things, just a daughter who cared for a mother who sadly succumbed to COPD earlier this year.
I send you my very best