When everyone is stuck on Tatooine, Qui-Gon has Obi-Wan analyze Anakin's blood, by Qui-Gon placing it inside his communicator. What actually happened?
He didn't just put the blood into any old communicator - his com has a special attachment that is designed to analyze blood for a midichlorian count. It does the reading locally, and transmits the results over the data line to the other end. This could be used by, for example, a Jedi in a hospital nursery to take samples from a large number of newborns as part of a standard screen, reporting the results to a central server for processing.
Essentially, it seems that most Jedi (or perhaps only those of Master rank) have coms that allow them to do on-the-spot blood analysis. It's perfect for when you are dispatched to remote worlds to search for new Jedi potentials. If it's cheap enough, there's really no reason NOT to give all your Jedi the ability to check people for potential - the Force works in mysterious ways, and you might just end up on a rink-a-dink backwater and stumble across a strong power in a young child.
When I visited Star-Wars Identities : The Exhibition I took this ominous photo :
And here is the legend :
So, according to this, it's this machine that did the test.
This space has been left intentionally blank.
It just did, OK.