Kenneth Tan Medical Clinic · Clinical Tools

HLA-B*5801 Decision Aid

For healthcare professionals

Before starting allopurinol: a Singapore shared-decision pathway for HLA-B*5801 testing, using the RASHES risk framework. Carriers face an ~80–100× risk of severe cutaneous adverse reactions (SCAR).

Patient profile

RASHES risk amplifiers (tick all present)

The other RASHES factors — high Starting dose and rapid Escalation — are prescriber-controlled and mitigated by start-low-go-slow.

Recommendation

The pathway

Test offered & taken
Positive → avoid allopurinol → febuxostat (40→80 mg).
Negative → allopurinol, start-low-go-slow.
Test declined / unavailable
Shared decision: febuxostat (test-free), or allopurinol with risk mitigation + explicit SCAR counselling.
Lower-risk, not tested
Routine testing not required (ACE/HSA). Allopurinol start-low-go-slow + SCAR counselling.

Allele prevalence (Singapore, approx.)

Chinese~20% (1 in 5)
Malay~7% (1 in 15)
Indian~4% (1 in 25)
Overall~18.5%

APLAR 2021 suggests testing where prevalence ≥5% — Singapore exceeds this. ACE/HSA keep testing targeted, not universal (PPV ~2%; not cost-effective universally — Dong 2015).

SCAR — counsel every patient

Stop allopurinol immediately and seek care if, especially in the first 8 weeks:

  • Skin rash, blistering or peeling
  • Mouth, eye or genital ulcers/soreness
  • Fever, facial swelling, feeling generally unwell

SCAR (SJS/TEN/DRESS) incidence ~3 per 1,000 on allopurinol. HSA: 80 cases Mar 2016–Oct 2021, 6 fatal; ~⅔ aged ≥60, ~⅔ with renal impairment.

Counselling script

"Allopurinol is our best long-term gout medicine, but about 1 in 5 Chinese Singaporeans carries a gene that raises the risk of a rare but serious skin reaction. We can do a one-off blood test for that gene before starting, or use an alternative (febuxostat) that doesn't need the test. Either way I'll start low, go slow, and you must stop and call us if you get a rash, mouth sores or fever in the first couple of months."

HLA-B*5801 testing in Singapore

  • Where: KK Women's & Children's Hospital (DNA Diagnostic & Research Lab), Tan Tock Seng Hospital (Molecular Diagnostic Lab), HSA Tissue Typing Lab.
  • Turnaround: ~1–7 working days.
  • One-off test (genotype doesn't change). Not needed if already tolerating allopurinol >3 months.
  • Cost/subsidy: varies — confirm current pricing; MediSave/CHAS may apply to the consult.