The API GH test kit has been disappointing for me. The first one I bought I thought worked well, but I had no idea if it was accurate. After a year or so, it ceased to work. The second one I bought didn't work at all. But, the KH test kit seems to work well and consistently, but still with unknown accuracy. That is to be expected - no hobby test kit can be assumed to be accurate, and no test kit costing whatever it costs can be guaranteed to be accurate without calibrating it yourself.