Click on Low Carb Tips on the orange menu bar, and read the article Ketosis and Ketone Testing Strips. I've included additional links at the bottom of the article as well.

Many diabetes patient-care sites are recommending the use of foil-wrapped ketostix (mega-espensive!! ) .... because there is a very high incidence of inaccuracy in regular ketostix (the chemical reagent in the pad degrades rapidly when exposed to air and moisture). The closer it is to 6 months since you opened the container, and the more humid your bathroom is, the greater is the likelihood the sticks aren't very accurate.

The other thing is accurate timing of the test. Use a watch. The reading is taken at 15 seconds. The pad will continue to darken over time, so that at 5 minutes, you may see a darker reading, but this is due to the breakdown of urine components when exposed to air. Any reading taken after 15 seconds is not accurate.

Doreen