Good question and one where we find a few errors!

The ICD-10-CM code Z21, Asymptomatic human immunodeficiency virus, is used when there is no documentation of symptoms, or if the patient is described as HIV positive, having known HIV, or similar terminology.  Coding guidelines state that if the provider documents AIDS, or if the patient is treated for any HIV-related illness, the code B20, Human immunodeficiency virus [HIV] disease is used.

It is important to note that we don’t alternate between the codes, depending on whether or not the patient has an HIV-related illness on a given date of service.  Once the patient’s condition moves from Z21 to B20, we code B20 on any subsequent encounter in which the HIV is assessed.  Once the patient develops any HIV-related illness, the code Z21 will never again apply to that patient.