Retest at or after 6 months of age. I actually called the manufacturer of these at my previous clinic for this same scenario and that's what they told me. Ideally you'd test around 6-8 weeks (better for closer to 10-16) and retest again around 6-8 months.

A kitten that young can falsely test positive for it because of their mother's antibodies, or if they've been recently vaccinated for FeLV.

If the test is negative, I STRONGLY recommend you vaccinate them for FeLV and separate them from their mama as soon as possible.

When in doubt, however, always ask your vet. Each clinic may have different protocol when it comes to these types of cases.