A mutation that does not cause resistance by itself could lead to resistance when combined with other mutations. Also, resistance to one drug sometimes results in increased sensitivity to another. The most common way that genotypic testing is done uses a machine that reads the gene sequence of the protease and RT genes. The results are compared to an original, or not mutated, HIV gene sequence. Any mutations are checked against a list of changes known to cause drug resistance.
Most drugs follow a set pattern of resistance mutations.
There is a probe for each of the mutations known to lead to drug resistance. A third method, called GeneChip, uses a chip that has many markers built onto it. A blood sample is put onto the chip and it is passed through a scanner. The results are compiled by a computer which shows any mutations in the genes. The accuracy of the results does not seem to differ among the different types of tests.