They are ok in a pinch. But they are not the most accurate. When in doubt use a bulb type or a refractometer. They will generally be ok, if you are testing coolant made by the same company making the test strip, but I have seen very odd numbers when trying a test strip on a different brand of coolant. Maybe the add pack has a different ph or something.
I use CTS for DCA but NOT for antifreeze,use a regular tester bought at any auto parts store for that.
i found when doing my DCA tests and then compare the freeze results from the test strips the test strips were waay off like 5 deg or more!
don't trust them.