Yeah, they're not perfect unfortunately and I've heard Metrix had a bad batch a year or two ago, and are notorious for being much less reliable the closer they get to their expiration date. Though I do hear they've gotten better and will replace faulty tests, there's an error code or something to let you know.
PlusLife are supposed to be much more reliable, but so freaking hard to come by and i can't even afford the $300 testing machine. 
Good luck in your hunt and if happen to find something, please update your post. I'll be sure to keep an eye out. 