When a strong positive is to be detected, almost all conjugates are bound to viruses and remain on the specific antibody (lower line) coated onto the nitrocellulose, giving rise to a strong dark-red signal. As a consequence, quantity of remaining gold conjugate that should come to react with the control reagent (upper line) is not enough to give rise to a visible signal. Thus control line appears too weak or even sometimes is not visible by eyes. Even if the control line is very weak, the test is regarded as valid.