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Whenever you purchase a new pulse oximeter, you should check for its accuracy before relying on it totally.
The first step of validation is to take a reading and see if the result is reasonable or not. Make sure you follow the instructions and take all precautions. For example, if it shows under 80% and you are feeling perfectly OK, the oximeter has a problem. Also the reading should be stable if you are resting and not moving.
The second step is to compare with another one. Take a reading with two oximeters at the same time and if the readings differ by less than 2-3%, then the oximeter should be fine. If the readings differ by more than 8-10%, then something must be wrong. However if the difference falls between these ranges, it means that the test is inconclusive.
Because of the nature of the technology, oximeters have an AVERAGE error margin of 2-3% when compare with readings obtained from blood samples. The word average is emphasized here because the error margin is normally lower (about 1-2%) when the oxygen level is higher than 90% and then goes down to 4-5% when the level is below 80%. A pulse oximeter, regardless of its manufacturer or cost, only gives an indirect estimate of the blood oxygen saturation level. The error margin always exists and is dependent on the range.
For any two good "accurate" oximeters, it is possible that their readings differ by 5+%. Assuming the true reading is 89%, it is possible than one reports 92% and the other reports 87%. Both readings are good because the readings are within their margins of error.
If in doubt, get two or more oximeters from different manufacturers and compare the average of their readings with yours.