Scenario 5: Ankylosing Spondylitis (AS)

Scenario 5: Ankylosing Spondylitis (AS)
A 32-year-old Caucasian male presents to the office with complaints of back pain, stiffness, especially in the morning, interrupted sleep due to pain, and difficulty in leaning over to tie his shoes. The patient first noticed these symptoms about 6 months ago but attributed them to his weekend basketball team’s games. He said he is exhausted due to sleep interruption. He has taken acetaminophen with some relief but says the naproxen seems to be working better. Married with 2 small children and works as a bank manager. Physical exam: Lungs clear but decreased chest excursion noted as well as decreased range of motion of hips and forward flexion, rotation, and lateral flexion restricted. Spine radiographs in the office revealed a slight kyphosis along with ankylosis at L5-S1. The APRN suspects the patient may have ankylosing spondylitis (AS). The APRN orders laboratory tests including an HLA-B27.
Question:
Why did the APRN order an HLA-B27 lab? How would that lab result assist in understanding what ankylosing spondylitis?  
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