Aren't test strips used mostly on HD coolants, with the main component being tested nitrites, which are not that important in automobile coolants?
Uaual test for automobile coolants is to check the glycol concentration, which can be done with an inexpensive hydrometer. Other than that you just go by time. After a couple of years there is a tendency for the silicates and phosphates to precipitate out, so the coolant should be changed every two to three years. Test strips are not going to tell you that silicates and phosphates are coming out of solution.