Cd 21 blood test question

Sara • Wed, Feb 17

I have an appt with my gyno tomorrow and I'm assuming she will send me for a blood test on cd 21. In the past I have had positive opks later in my cycle but recently they have been on cd 13. Will a cd 21 be accurate if I ovulate later then cd 13/14? This is the cd I got a positive opks and the cycle length