Amelia,
The results are easy to read, first establish a baseline, preferably early in the treatment. Subsequent tests will either show a decrease in circulating HER2 cells if your current treatment is working, an increase if your treatment is ineffective or no change if you are stable.
If the test measures below 15 ng/ml you are considered in remission, above, means cancer activity. The test should be administered about every 3-4 months.
For more info:
http://www.her2support.org/serum.htm
Regards
Joe