Discover how HLA-B27 positivity influences ankylosing spondylitis risk by age group and sex, with clear prevalence data and clinical insights.
Among HLA-B27 positive individuals, the lifetime risk of developing ankylosing spondylitis (AS) is about 5–7% overall, but rises to 20–30% in first-degree relatives of AS patients. AS typically begins between ages 20–40, with peak incidence in the 20–30 year range. Globally, AS affects about 0.1–1.4% of the population, with men 2–3 times more commonly affected than women
📊 Age-wise Incidence in HLA-B27 Positive Patients
- 20–30 years:
- Peak onset period.
- Most AS cases begin in this decade.
- Rough estimate: ~60–70% of AS patients report first symptoms here.
- 30–40 years:
- Still a high-risk window, but incidence declines compared to the 20s.
- About 20–30% of cases begin in this age group.
- 40+ years:
- New onset is rare.
- Less than 10% of cases start after 40.
👉 Interpretation: If someone is HLA-B27 positive and symptom-free by their late 40s, the likelihood of developing AS later is very low.
🌍 Population Prevalence
- General population prevalence of AS:
- 0.1–1.4% worldwide, depending on ethnicity and HLA-B27 frequency.
- In India, prevalence is estimated around 0.2–0.5%.
- HLA-B27 prevalence:
- Varies widely: 6–9% in Europeans, 2–6% in Indians, <1% in Japanese.
- Not all carriers develop AS — most remain healthy.
👩🦰 Sex Differences
- Male-to-female ratio:
- Historically 2–3:1, though newer imaging shows underdiagnosis in women.
- Men tend to have more severe radiographic progression.
- Women often present with milder or atypical symptoms, leading to delayed diagnosis
🧾 Summary Table
| Factor | Data / Estimate |
|---|---|
| Lifetime risk of AS in HLA-B27+ | ~5–7% (general), up to 20–30% in relatives |
| Age of onset peak | 20–30 years |
| Onset 30–40 years | ~20–30% of cases |
| Onset >40 years | |
| Global prevalence of AS | 0.1–1.4% |
| Prevalence in India | ~0.2–0.5% |
| Male-to-female ratio | 2–3:1 |
✅ Key takeaway: If an HLA-B27 positive patient is in their 20s–30s, that’s the highest-risk window for AS onset. By 40+, the risk of new onset drops sharply. Overall, AS remains relatively uncommon, affecting less than 1% of the population, with men more frequently and severely affected.
