A faint test line still indicates negative results. The presence of a second test line, regardless of how faint, indicates a negative result. Some test lines tend to naturally run lighter than others. Even if there is some amount of a drug in the urine, if there are two lines present, the urine does not have the minimum amount of drug needed to consider it positive, so it is considered negative.
With some drugs, the minimum amount is relatively high, like amphetamines (1000ng/mL). With other drugs the minimum amount is relatively low, like marijuana (50ng/mL).
