High Urea Levels in Urine & Heart Worm

Peluche had blood work done a while ago and it showed high urea levels. I found some natural supplements that can help lower the levels; cinnamon, Chinese rhubarb, and stinging nettle seed. Does anyone have experience with this or know of anything else that can help with his kidneys?

Also, how long should I wait until doing another heartworm test? He tested positive for HW this year and I took him to a holistic vet that gave him a medication to help kill them off. It's been approx. 6 months since he was last tested.