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 a 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.