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HLA-B*57:01 Testing Before Abacavir
If you're ever prescribed the HIV drug abacavir, one blood test should come first. It looks for a single gene variant, HLA-B*57:01. Carriers, about one in fifteen people of European descent, have roughly a 50% chance of a whole-body reaction within two weeks, and on a second dose it can turn fatal. The test costs $50 to $200, comes back in days, and is good for life. It quietly erased one of HIV medicine's worst side-effects, and became the model every other pharmacogenetic test copies.
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The reaction was the problem, and the gene was the answer. Before testing, 5% to 8% of European-descent patients on abacavir got a high fever, a body-wide rash, and GI symptoms in their first two weeks. Almost every one of them carried HLA-B*57:01. Screen for the variant and steer carriers to another drug, and confirmed reactions drop from 2.7% of patients to zero 1. The marker holds equally in Black and white patients 2.

The variant is not a risk factor, it's a switch. Abacavir slips into a pocket on the HLA-B*57:01 protein and nowhere else, changing which bits of your own cells the immune system displays; it reads the new set as foreign and attacks 3. No variant, no pocket, no reaction.

This is a decision, not a routine. The only action is to make sure the test happens before the drug does.

For most people, nothing visible happens. A negative result files itself into your record and abacavir starts. For carriers, the morning the result comes back positive is the morning a different drug gets prescribed and a reaction that would have hit you never does. At the population scale, a whole category of severe drug reaction stopped happening in HIV clinics. And the test wrote a playbook: screening for DPYD before chemotherapy, HLA-B*58:01 before allopurinol, HLA-B*15:02 before carbamazepine all follow the arc this one ran first.

Cash price runs $50 to $200, but almost nobody pays it: insurance, Medicaid, Ryan White, the NHS, and most public payers cover it, because one hypersensitivity hospitalization costs far more than screening everyone 5. Everyone gets tested regardless of ancestry โ€” self-reported ancestry is unreliable and guessing wrong is expensive 6.

The fine print โ€” when to skip it, and what people get wrong

A negative test doesn't clear every early fever or rash; non-carriers still get unrelated illnesses, and clinicians still pause the drug if the picture is unclear. And it's not "just a rash" โ€” it's a whole-body hypersensitivity reaction.

The lethal case is re-challenge: a carrier mistakes the reaction for a stomach bug, stops, restarts, and blood pressure crashes within hours. That drove abacavir's boxed warning 4; a single dose in a confirmed carrier is never given.

References
  1. 1Mallal et al. (2008). HLA-B*5701 screening for hypersensitivity to abacavir (PREDICT-1). New England Journal of Medicine. link
  2. 2Saag et al. (2008). High sensitivity of human leukocyte antigen-b*5701 as a marker for immunologically confirmed abacavir hypersensitivity in white and black patients (SHAPE). Clinical Infectious Diseases. link
  3. 3Illing et al. (2012). Immune self-reactivity triggered by drug-modified HLA-peptide repertoire. Nature. link
  4. 4FDA (2008). FDA labeling change for abacavir: HLA-B*5701 screening recommended prior to initiation. link
  5. 5Schackman et al. (2008). The cost-effectiveness of HLA-B*5701 genetic screening to guide initial antiretroviral therapy for HIV. AIDS. link
  6. 6Martin et al. (2014). Clinical Pharmacogenetics Implementation Consortium guidelines for HLA-B genotype and abacavir dosing: 2014 update. Clinical Pharmacology & Therapeutics. link
Related in the handbook (4)
Related
  • โ€” Like HLA-B*57:01, G6PD status is a one-time genetic check that flags drugs to avoid.
  • โ€” HLA-B*57:01 is a clear instance of genetic testing actually altering what you're prescribed.
  • โ€” Like HLA-B*57:01 before abacavir, HLA-B27 is a one-and-done HLA test for a specific question.
  • โ€” The abacavir gene test is the textbook pharmacogenomic win โ€” one gene checked before one drug.
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