There are two main reasons why you may get a nitrate reading in your reverse osmosis water.

1. You have high nitrate in your tap water and even at 95-98% removal this still leaves a reading. Some areas of tap water do have high levels of nitrate and this can affect the end product of RO water. People believe that RO units will strip every molecule of nitrate from the water despite whatever reading the tap water is. In high nitrate areas some nitrate will be left in the RO water. A membrane has an approximate removal rate from between 90 - 98% but this can be affected by the mains water pressure, how old the membrane and carbon/sediment filters are and the temperature of the feed water. Use of DI resin can help in this situation.
2. The test kit that you are using will give a false reading with RO water. We recommend that you prepare a salt mix ready for use and then measures the nitrate after the salt has dissolved fully. The salt remineralises the RO water and this in turn stabilises the water. In most cases you will find that no nitrate is present after adding salt.