The digitals are less sensitive- 40 miu, i think the opks they send out on ebay are generally 20 miu.
If you have a look on ebay they sell less sensitive ones, 30 miu and 40 miu. A normal surge is above 40 so they will still pick up your surge but you probably wont get that couple of days fade-in where your tests get darker closer to O, but on the up side you might not get such confusing opks results.
Does that make sense? Say your body is producing about 18, a normal 20 miu test will look close to + when a 30 or 40 miu would still look obviously negative xx