Advice regarding testing

These diagrams from El Pais illustrate what a lateral flow test will indicate at different stages of a Covid infection. 

A red line at the point C shows that the test has been conducted correctly. A red line at the point T shows that you are infected and infectious. 

In the early stage, you may have symptoms but still test negative i.e. there is no red line at point T. You should still isolate and take another test later. If the line at T is feint, then you are either in the early or late stages of infection. At the height of your infection, both lines will be clearly visible. 

In Spain, isolation is seven days from the onset of symptoms or a positive test, If you have been symptom free for three days then your isolation can end otherwise your isolation is for ten days.