Resistance Tests— These give the lowdown on how different HIV meds might work against your virus, and on how the virus has changed, or mutated, to duck your current drugs. The HIV-resistance test has two varieties: the genotype and the phenotype—often used in tandem.

Genotype— This test scours your HIV for actual mutations, many of which disarm certain meds against your virus. Others may sensitize your virus to some meds, which means those pills are working overtime for you. But most mutations are more enigmatic: They interact with each other and must be analyzed as part of a bigger picture. By knowing your mutations, a savvy doc can better predict which meds may or may not work for you. The genotype test is used more than the phenotype, partly because it’s cheaper and speedier. But it measures resistance indirectly, so interpreting the results can be tricky.

Phenotype—This one charts how your HIV responds to specific drugs, pitting it directly against them one at a time—it measures how effectively the virus is kept from infecting cells in the presence of each drug. Each med gets a number value based on how well the virus performed; the higher the number, the more resistant your virus is to a particular med. It measures more directly than a genotype, making it especially useful for those who’ve experienced many meds. But it costs more, takes longer—and still demands artful interpretation.