Ankylosing spondylitis is not as rare as many people assume. The broader category of spondyloarthritis, which includes AS, affects roughly 1% of U.S. adults, a prevalence similar to rheumatoid arthritis. That translates to an estimated 1.7 to 2.7 million Americans between the ages of 20 and 69, depending on which diagnostic criteria are used. Still, because it’s frequently misdiagnosed or diagnosed late, it can feel rare to the people living with it.
How Common AS Actually Is
Globally, AS prevalence varies enormously by population. Reported rates range from about 7 per 100,000 people in Japan to 540 per 100,000 in Turkey. In the United States, national survey data from NHANES puts the prevalence of spondyloarthritis at 0.9% to 1.4% of adults. For context, rheumatoid arthritis has a similar prevalence range of roughly 0.2% to 1%, meaning AS-related conditions are about as common as the most well-known form of inflammatory arthritis.
So while AS doesn’t qualify as a “rare disease” by standard medical definitions (which typically require fewer than 200,000 affected people in the U.S.), it’s still uncommon enough that many primary care doctors see relatively few cases. That gap between actual prevalence and clinical awareness is a big part of why diagnosis takes so long.
Why It Seems Rarer Than It Is
The median time from first symptoms to an AS diagnosis is about 6 years. That number has actually gotten worse over time, not better. In 1998, the median delay was 3.6 years. By 2017, it had more than doubled to 8.3 years. Women face even longer waits, with a median delay of nearly 7 years compared to about 5.7 years for men.
Part of the problem is that early symptoms, particularly chronic low back pain and stiffness in young adults, overlap with far more common conditions. Many people cycle through general practitioners, orthopedists, and physical therapists before ever seeing a rheumatologist. During those years, they don’t appear in prevalence statistics for AS, which makes the condition look rarer in medical records than it is in reality.
Adding to the confusion is a related condition called non-radiographic axial spondyloarthritis. This causes the same inflammatory back pain as AS but doesn’t yet show visible joint damage on X-rays. Almost no population-level prevalence data exists for this form, meaning a significant number of people with the same underlying disease process simply aren’t being counted.
Who Gets AS
Symptoms typically begin in the second or third decade of life, with a median age of onset around 26. About 92% of people develop their first axial symptoms before age 45, and 95% before age 48. This early onset is one of the distinguishing features of AS: chronic back pain that starts in your 20s or 30s and doesn’t improve with rest should raise a red flag.
For decades, AS was considered overwhelmingly a male disease, with widely cited ratios of 3 men for every 1 woman diagnosed. Newer research challenges that assumption. A study of U.S. Army personnel found that AS incidence was essentially the same in men and women, with no statistically significant difference between the sexes. The old male-heavy numbers likely reflect referral bias and diagnostic tools that caught the disease later in its course, when structural damage visible on X-rays is more advanced. Women may present differently or be referred to rheumatologists less readily, which delays their diagnosis even further.
The Role of Genetics
About 6% of U.S. adults carry a gene called HLA-B27, which has been linked to AS since the 1970s. Carrying this gene substantially increases your risk, and the vast majority of people with AS test positive for it. But most people who carry HLA-B27 never develop the disease. The gene is a risk factor, not a guarantee, and doctors use it as one piece of the diagnostic puzzle alongside imaging and clinical symptoms.
Because HLA-B27 frequency varies by ethnicity and geography, so does AS prevalence. Populations with higher rates of HLA-B27 tend to have more AS. This partly explains the wide global variation in how common the condition is from country to country.
Uncommon, Not Rare
The most accurate way to think about ankylosing spondylitis is that it’s uncommon but not rare. It affects a similar share of the population as rheumatoid arthritis, yet receives far less public awareness. The long diagnostic delays, the historical undercounting of women, and the near-total absence of data on its non-radiographic form all contribute to a perception of rarity that doesn’t match the actual numbers. If you’re experiencing inflammatory back pain that started before age 45, improves with movement, and worsens with rest, those symptoms are worth discussing with a rheumatologist regardless of how “rare” the condition sounds.