The Serum Of A Client Was Used For Hiv Test

the serum of a client was used for hiv test with alere determine it came out with a red control line and a faint line after some minutes and when buffered it came out with only one red line.to further confirm the result, a unigold confirmatory kit was used it came out with one red line.my question now is why the faint line when the serum was used?