How accurate are Jungle Quick Drip test strips?

I've been using them for the last couple weeks on my turtle tank, but the measurements never seem to change. Earlier today (about 3 hours ago) the strip showed 2.0ppm nitrite levels. I did a 50% water change, and now, 3 hours later, the strip still shows about 2.0ppm. It is somewhat difficult to tell with a pale pink, bit I was wondering if anybody else had similar experiences.