Question: I noticed that some evaluation and management (E/M) codes have a plus symbol (+) in front of them. I was wondering what this means? Texas Subscriber Answer: As an example, take a look at the following prolonged services codes. You will see that they all have the + symbol in front of them: When you look at codes +99354-+99357, you will notice the + symbol, which means these are add-on codes. An add-on code is a code that cannot stand alone, explains Linda Martien, CPC, COC, CPMA, AAPC Fellow. It is always used with or added to the base code. For instance, 99214 (Office or other outpatient visit for the evaluation and management of an established patient, which requires at least 2 of these 3 key components: a detailed history; a detailed examination; medical decision making of moderate complexity …) would be your base code and +99354 would be the add-on code if the provider spent 30 minutes to one hour in prolonged service, according to Martien.