For a 'good test' taking a sample should be easy and ideally quick with a high predictive value. No test is perfect, but in response to COVID-19 we need a test that has a high probability of being right. With a test we also want few false positives (people who test positive but don’t have the virus) so isolation is not done unnecessarily Most importantly we want as few false negatives (people who test negative but actually have the virus) to avoid risking more unintended infectious exposures. In medical speak a '‘good’ test is one with high sensitivity: that is a test that returns few false negatives. As the Chief Medical Adviser to the UK government, Prof. Chris Whitty, said, ‘the one thing that is worse than no test is a bad test’.