I'm prepared to be shown to be wrong here but I think it might be something to do with whether or not there is a mismatch between your and your baby's blood type.
Most of us are Rhesus positive (there is a particular factor present in the blood). A minority are Rhesus negative (said factor is absent). When the mother is positive and the baby negative (or vice versa) the mother's body can react as though the baby is "foreign" and the immune system produces antibodies to attack it. If the antibodies are present in the mother's blood, it could indicate a potential problem.
That said, I'm pretty vague about the details. Please, please, please ask your MW to explain it to you (rather than just tell you what it is not!).
You can get funny anti-bodies in your blood not to do with infections.
Stuff like anti-e, anti-c, anti-k. They can cause a rhesus incompatibility even if you're rhs pos. Normally just means you have to have extra blood tests to check titre levels and doesn't normally cause a problem.
I have Anti M in my blood - no one is sure what it is, where it came from, why I have it etc etc. All they do know is it doesnt do any harm to the baby and certainly doesnt do any harm under a certain level.
What it means for me is really annoyingly having to go and get blood taken every 4 weeks just to check that it hasnt risen to a level where there is a slight possibility that it may possibly lead to jaundice in the baby.
First time around they were more cautious with it - this time didnt even flinch when i enquired about home birth. None of my midwives or consultants have ever heard of anyone else with it or any negative cases associated with it.
If you are coming back as inconclusive it probably means that you may have a slight trace of an antibody - a level so low they can barely measure it and if it it this low at 28 weeks (and they are prepared to wait another 4 - 6 weeks to retest) then its unlikely its going to do any harm. Some of the other antibodies can be a little more dangerous in that they could cause jaundice in the baby - but thats a could situation and not a definite. If they arent worried then I wouldnt be either - they might make you see a consultant - who probably wont know anything either lol
On the plus side (I think!) I had a total needle phobia at the start of my first pregnancy. I used to shake and cry at the thought of blood being taken. Combined with other complications needing blood tests and all of this I am now completely cured and barely blink when they take it.
Every cloud has a silver lining and all that!
Hope everything sorts itself out for you - really doesnt sound as if they are worried. When I had my first positive screen they sent me to the hospital the very next day as an emergency so if they are happy for you to wait 4 - 6 weeks (and they always over react if they can in my experience) then sounds like nothing to worry about.