Take a look at the guidance for a patient who is able to measure the oxygen level in their blood after slipping their forefinger in a pulse oximeter that they have on standby.
It gives very clear information about allied symptoms such as breathlessness or coughing that the person might have – and when they should worry and seek medical help or stay at home and see how their condition goes.
https://www.england.nhs.uk/coronavirus/publication/pulse-oximetry-to-detect-early-deterioration-of-patients-with-covid-19-in-primary-and-community-care-settings/