Resistance of HIV to antiretroviral drugs is an important cause of treatment failure, which is covered in detail in the section entitled Virological tests for diagnosing and monitoring HIV infection. Acquisition of drug resistance mutations depends on the interplay between factors such as patient adherence to treatment, drug potency, genetic barrier to resistance, host genetics, and specific mutations contributing to viral drug resistance and replicative capacity43. Rates of drug resistance vary significantly between populations, but the rates of transmitted mutations are still shown to be significant in representative studies44. In ART-naïve patients, studies have shown that the presence of drug resistance mutations may reduce the efficacy of first-line or subsequent regimens, increasing the risk of developing further resistance, and is associated with more rapid disease progression45-47. Although rates of drug resistance in transmitted virus are currently low in Australia48, it is the recommended standard of care that HIV drug resistance testing is undertaken at the time of all new diagnoses of HIV infection, including during PHI, not only to guide choice of ART but also to provide important public health surveillance information regarding population rates of transmitted drug-resistant variants.