If you are on municipal water, the analysis should include a test for lead, chlorine, fluoride, nitrates, and hardness. Other potential contaminants can be ascertained by contacting your local water company, which is required to disclose the results of their tests to customers who ask for them. If you are on a private well or water system, the analysis should add tests for pH, iron, sulfates, biological contaminants, herbicides, and pesticides. If your location means that your water could be contaminated with radon, toxic waste from landfills, heavy metals like mercury, cadmium, and arsenic from mine tailings, or leaking fuels and oils from underground storage tanks, it would be very wise to have appropriate tests done for those problems as well. Don't assume that the water coming out of your tap is safe just because it has been tested.
Nitrate levels can fluctuate widely according to season.
There are some biological contaminants like Cryptosporidium and Giardia that are not affected by chlorination, and routine tests for these and many other contaminants are not yet required by the EPA or regularly performed by water suppliers. Pay particular attention to aluminum, arsenic, biological contaminants, chlorine, fluoride, hydrogen sulfide, hydrocarbons, nitrates, and radon, because they are most dangerous to your health. Also pay attention to pH, hardness, manganese and iron. While they are normally not harmful to health, they affect the kind of technology necessary to solve your water problem. If you decide to purchase a filtration system, be clear about the pros and cons of each technology, the cost to purchase, install, and maintain it, and its projected life.