I recently bought an oxymeter and have tried it on several members of the family including myself, I was a bit concerned that my mothers reading varies between 87% and 90%, the rest of us vary between 95% and 98% she does have a residual cough that she had checked with a chest xray and a spirometry and was normal but otherwise in good health, should I be concerned?
Try looking at her not the numbers. If she isn't breathless when doing her day to day stuff her lips aren't going a blue colour check her hands. If they are pale and cold get her to wear warm gloves for a while and exercise her hands then try again. Oximetry is inaccurate in old people with poor circulaton.