Genetic markers

Over a decade ago, scientists found that a 32-base pair deletion in the gene for CCR5 has a protective effect against the development of HIV. Individuals carrying this deletion are either heterozygous (deletion from one parent) or homozygous (deletion from both parents) for delta 32. Testing for this deletion is normally done only in research settings. See Receptors, co-receptors and immunity to HIV for further information on this topic.

A close association between abacavir hypersensitivity reaction and the presence of the class 1 HLA-B*5701 allele has led to recommended screening before starting an abacavir-containing regimen.^{1 2} 

Anyone who is positive for HLA-B*5701 should never take abacavir and this status should be recorded in the medical chart. A negative test does not rule out the possibility of a hypersensitivity reaction, so patients about to begin abacavir therapy should be informed about possible reactions to the drug.

If a reaction is to occur, it will usually happen within the first six weeks of treatment. The risk of occurrence is higher in white patients (5 to 8%) than it is in black patients (2 to 3%).² A negative skin patch test for HLA-B*5701 does not rule out a need for clinical vigilance for hypersensitivity reaction.^{3 4 5}