Can nitrofurantoin cause white blood cells to be low along with 16 hour fast only had two doses before cbc test and it was slightly low at 4.1 normal is 4.5-11 last year white blood cells was 7.4 which is a big difference?

This actually is not a very big difference and probably not important; your WBC may just run on the low side. Also, some population groups are known to have lower counts than others, especially people of African descent -- but the standard normal values are biased toward caucasian populations. In any case, nitrofurantoin is not a likely cause. Discuss with your doctor.