Is it true that a nst will only detect ischemia greater than 65 to 70 percent? if so isn't this a false sense of security when test is normal?

Ischemia: Blockages greater than about 70%, can impair the flow of blood and cause ischemia. If ischemia is detected on a stress test, it signifies significance of the blockage regardless of percentage. A negative stress test does not eliminate the possibility of ischemia completely, however has a very good negative predictive value especially if done with imaging.

Depends on pt: You will have only symptoms from cad, if your blocked arteries are >70% blocked. But people can have heart attack from plaque rupture from blockages <70%. Thats why treating risk factors are very important.