Seeing one line on your test does not indicate a defect. This is a valid result, and is usually reflective of very low or very high levels.
A defective test would show no lines at all. The most common reasons this can happen are:
Not enough saliva: Try to generate as much saliva as possible onto the test sponge.
Test strip not fully pulled: Be sure to pull the test strip until the blue dot is fully revealed at the end.
Dry Mouth: If your mouth feels dry, sip a small amount of water before taking the test.
Before the test: Avoid eating, drinking (except water), smoking, or brushing your teeth for 30 minutes before the Eli test. Do not take biotin supplements beforehand, as these can affect test accuracy.