However, instead of waiting for the results, she started to feel better and travelled all the way down to London to attend Parliament, where she was a participant in a debate about the Covid-19 pandemic.
After that, she received her positive result and 'without seeking advice' travelled all the way back up the way to Scotland, where she is now in self-isolation.
She has been suspended by her party following her admission, and has reported herself to the police.
Ian Blackford, the leader of the SNP in Westminster, said: "I have spoken with Margaret Ferrier this evening, who fully accepts that what she did was wrong.
"Margaret will be referring herself to the parliamentary standards commissioner as well as the police.
"I am tonight suspending the whip from Margaret."
In a statement earlier this evening, Ferrier said: "There is no excuse for my actions."
She added: "I apologise unreservedly for breaching Covid-19 restrictions by travelling this week when I shouldn't have.
"On Saturday afternoon, after experiencing mild symptoms, I requested a Covid-19 test which I took that day."
"Feeling much better, I then travelled to London by train on Monday to attend Parliament as planned.
"This was wrong, and I am very sorry for my mistake. I travelled home by train on Tuesday morning without seeking advice.
"This was also wrong and I am sorry. I have been self-isolating at home ever since.
"I have used Test and Protect and I have notified the House of Commons authorities who have spoken with Public Health England. I have also notified the police of my actions.
"Despite feeling well, I should have self-isolated while waiting for my test result, and I deeply regret my actions.
"I take full responsibility and I would urge everyone not to make the same mistakes that I have, and do all they can to help limit the spread of Covid-19."
Ian Murray, Labour's Shadow Secretary for Scotland, said: "This is astonishing recklessness from an SNP MP, which has put people's health at risk.
"Through her irresponsible actions, she very possibly has passed on the virus to a vulnerable person, who may now have Covid-19 and be in danger. She has put passengers, rail staff, fellow MPs, Commons staff and many others at unacceptable risk.
"To breach the rules twice is simply unforgivable, and has undermined all the sacrifices made by her constituents.
"Nicola Sturgeon must come out and condemn her MP's actions and tell the Scottish people what disciplinary action she will be taking. There cannot be one rule for Margaret Ferrier, another for everybody else."
Chosen for YouChosen for You
Most Read StoriesMost Read