My latest gadget is an oximeter and it says in the instruction for blood oxygen levels, 95% to 100% = Good, 90% to 94% OK but think about seeking medical advice and anything below 90% to seek medical advice immediately.
First couple of days reading I was at 97% oxygen level so fine but yesterday had a shock to see a reading of 86% but then after panicking I notice the display inverts itself every time you press the on button and the display was upside down so true reading was 98%. ?