My son has just done an LFT. He has had classic cold symptoms with a sore throat which has now eased but now has a bunged up nose . No temperature , continuous cough or loss of taste or smell so NHS says he doesn't need a PCR. When doing the LFT, there was blood on the swab after removing it from his nose, probably because of his cold. I assume that would make the test invalid? I'm waiting for the result but wondering if it will say that it's an invalid result or if it could give a false negative . Anyone had this ?