I guess you may have misunderstood these tests. CO2 is not included in the GFR test. The GFR test is a kind of test that measures your kidney function. It should be generally higher than 90ml/min/1.73m^2. If your GFR is less than 60, then it is a concern.
The CO2 is generally measured with a blood gas test. The normal range for CO2 is about 21 to 31. A level of 19 is very close to normal values and it is per se not a concern.