STOP SJS

Find Care

Genetic Testing

For a small number of drug and ancestry combinations, a blood test before the first dose meaningfully lowers the risk of SJS/TEN. For most people and most drugs, no such test exists.

What is actually being tested

The variants involved are HLA alleles — versions of genes that help the immune system decide what is foreign. Certain alleles change how a particular drug is presented to the immune system, and in people who carry them, that specific drug is far more likely to trigger a severe reaction.

The association is narrow. It is not a general SJS/TEN risk score. It is always a pairing: this allele with this drug. A negative result for one pairing says nothing about any other medication.

The pairings that have guidelines behind them

  • HLA-B*15:02 and carbamazepine — strongly associated in people of Southeast Asian ancestry, including Han Chinese, Thai, Malaysian, and some Indian populations. The FDA label for carbamazepine carries a recommendation to test people of those ancestries before starting it.
  • HLA-A*31:01 and carbamazepine — associated across a broader range of ancestries, including European and Japanese, with a different and generally lower risk profile.
  • HLA-B*58:01 and allopurinol — associated with severe reactions, and most studied in Han Chinese, Thai, and Korean populations.
  • HLA-B*57:01 and abacavir — testing before starting abacavir is long-established standard practice in HIV care.

CPIC — the Clinical Pharmacogenetics Implementation Consortium — publishes free, peer-reviewed guidelines on exactly what to do with each result. If you want one authoritative link to give a prescriber, that is the one.

Where testing happens

HLA typing is an ordinary blood or cheek-swab test run by hospital and commercial laboratories. It is ordered by a clinician — usually the prescriber, sometimes a clinical pharmacist, allergist, or genetics service — rather than sought out directly. The practical route is to raise it with whoever is proposing the medication.

Coverage varies. Where a test is named in an FDA label, insurers are generally more willing; where it is not, ask about cost before the sample is taken. Consumer ancestry tests are not a substitute — they do not report these alleles at the resolution clinical decisions need.

If I test negative, am I safe to take the drug?

No. A negative result lowers the risk substantially for that specific pairing, but SJS/TEN still occurs in people without the allele. Report any rash, fever, or mouth or eye soreness in the first weeks immediately, whatever the test said.

Is SJS/TEN hereditary?

Mostly, no — the great majority of cases have no identified genetic link. But HLA alleles are inherited, so where a case was tied to one of the pairings above, close relatives may share it. That is worth mentioning to family members who are offered the same drug.

Should I get tested for everything, just in case?

There is no panel that meaningfully covers “everything”. Broad pre-emptive pharmacogenomic panels exist and are used in some health systems, but outside those programmes the evidence supports testing around a specific drug decision. Discuss it with a clinical pharmacist if your health system has one.

Going further