I bought a test kit like this one. I've used it a few times and overall I think it's accurate, but only having a professional test would prove that I guess. It actually a fun little exercise - science experiment.
The most interesting thing that it has suggested is that with the 2 old small raised bed areas (6'x6') I have (1 for sweet pea flowers and 1 for tomatoes, peppers, beans or peas) they are deficiency in Potassium. From what I understand that could be a result of using the same area with the same crop or plant over and over. The output of these little raised bed has diminished over the last couple years.
I've been treating with Muriate of Potash this year, but not !00% if it's helping.There wasn't a lot of info on line on how to treat potassium deficiency in garden soil really. I'm also making an effort to improve the soil nutrients available to the plants overall in the vegetable area via some drip irrigation fertilization. I was late on the sweet peas, but I do have some tomatoes and all in and they seem to be doing better so far this year so we'll have to see.
I would appreciate your posting back the results, did they test 1 area or more, how comprehensive the test were (did they do more than just NPK), how much they cost... Very interested to see how it goes for you, I would like to do the same some time soon.