Though pulse oximeters do indeed measure blood oxygen levels and people infected with coronavirus or Covid-19 often to show low blood oxygen levels, it’s not proven whether a pulse oximeter is really a useful tool in self-detection. This is partially because low blood oxygen is a common symptom of numerous underlying medical issues and chronic illnesses. But it’s also because the medical community hasn’t had an extended period of time or sufficient data to come to any consensus on the matter, so for the time being, pulse oximeters should not be relied on for such a purpose. This is in keeping with the fact that fingertip pulse oximeters are not meant to replace professional medical opinions, to begin with.