HLA Associations of Drug Hypersensitivity
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Direct answer
HLA associations of drug hypersensitivity link specific alleles to severe skin reactions, the most important being HLA-B*1502 with carbamazepine. Carriers of the allele, most common in Han Chinese and some South-East Asian populations, face a much higher risk of Stevens-Johnson syndrome and toxic epidermal necrolysis, so screening is advised before carbamazepine where the allele is common. Pre-prescription tests also exist for allopurinol (HLA-B*5801) and abacavir (HLA-B*5701).
What you must remember
- Carbamazepine: HLA-B*1502 is the strongest association with Stevens-Johnson syndrome and toxic epidermal necrolysis, mainly in Han Chinese and some South-East Asian ancestries.
- Allopurinol: HLA-B*5801 is linked to severe cutaneous reactions, including SJS, TEN and DRESS, most strongly in Han Chinese, Korean and Thai populations.
- Abacavir: HLA-B*5701 carriers have a markedly higher risk of hypersensitivity, so testing before the first dose is standard and a positive result contraindicates the drug.
- Carbamazepine milder reactions: HLA-A*3101 is associated with maculopapular rash and DRESS in European and Japanese populations.
- Ethnic variation: allele frequency differs widely between populations, so the value of screening depends on ancestry.
- Mechanism: the allele shapes which drug-peptide complexes T cells recognise, which is why associations are drug-specific.
- Predictive limits: a negative result lowers risk greatly but does not exclude every reaction, so clinical monitoring continues.
Screening before prescribing
Pharmacogenetic screening is a check done before a drug is started, not a test ordered after a rash appears. The steps below show how it fits into routine prescribing of these three drugs.
- Identify the drug: check whether the planned drug has a recognised allele, such as carbamazepine, allopurinol or abacavir, before the first dose.
- Consider ancestry: allele frequency varies, so screening is most useful where the risk allele is common and testing is available.
- Send the test: HLA genotyping on a blood sample is done before the first dose, so the result is ready when treatment is decided.
- Interpret the result: a carrier of HLA-B*1502 or HLA-B*5801 is steered towards an alternative drug, and a positive HLA-B*5701 result rules out abacavir.
- Counsel the patient: explain the warning signs of rash even after a negative test, because the risk is reduced rather than removed.
- Monitor clinically: any rash, fever, blistering or mucosal involvement prompts stopping the drug and urgent review.
Where no test exists for a drug, careful clinical monitoring remains the main safeguard. Screening results should always be read together with the clinical picture.
Where the exam tries to catch you
Allele questions test which gene matters for which drug, and whether screening changes management.
- Trap: assuming HLA-B*1502 explains every carbamazepine rash Correct: it predicts severe SJS and TEN, while milder reactions are linked to other alleles such as HLA-A*3101.
- Trap: treating a negative HLA-B*5701 result as a complete guarantee Correct: the result makes abacavir hypersensitivity very unlikely, but patients are still counselled to report symptoms.
- Trap: using the abacavir allele to screen for allopurinol Correct: allopurinol is linked to HLA-B*5801, a different allele, so the correct test must be chosen for each drug.
Frequently asked questions
Which HLA allele is linked to carbamazepine-induced Stevens-Johnson syndrome?
HLA-B*1502 is the allele most strongly linked to carbamazepine-induced Stevens-Johnson syndrome and toxic epidermal necrolysis. It is found mainly in Han Chinese and some South-East Asian populations. Screening is advised before carbamazepine where the allele is common and testing is available.
Why is HLA-B\*5701 tested before abacavir?
HLA-B*5701 is tested before abacavir because carriers have a markedly higher risk of abacavir hypersensitivity. A negative result makes abacavir hypersensitivity very unlikely. A positive result means abacavir should not be given.
Which allele is associated with severe allopurinol skin reactions?
HLA-B*5801 is the allele associated with severe cutaneous adverse reactions to allopurinol, including SJS, TEN and DRESS. The association is strongest in Han Chinese, Korean and Thai populations. Screening in those populations is one strategy to reduce the risk.
Does a negative HLA test mean a drug reaction cannot occur?
A negative HLA test does not rule out every drug reaction, because most alleles raise risk rather than making reactions certain. Non-carriers can still develop hypersensitivity to carbamazepine, allopurinol or abacavir. Patients therefore still need clinical monitoring after the drug is started.