They're not accurate and expensive. They're suppose to measure ketones in the urine but are very hit and miss. A better and far less expensive way is simply to smell whether your urine has a strong strange smell (not like urine at all), this will give you an indication. If your ketosis is mild, as it tends to be with the Dukan diet you may not really be able to tell but it will still be a lot more reliable than those little sticks.
I've been using them for the past week, total waste of money as everytime i've used them (first thing in morning) they've shown just the mildest of ketone content (vaguely shows the second colour on chart) or no ketones at all (first colour), yet the weight is steadily coming off.
I'd agree with previous posts, save your hard earned cash and follow your nose instead! x