Which assay is positive in 95% of patients with Ankylosing spondylitis?

• A. CEA
• B. Philadelphia chromosome
• C. Ret-proto oncogene
• D. HLA B27

Answer: (D)

In the context of Ankylosing Spondylitis (AS), the assay that is positive in approximately 95% of patients is HLA B27. This genetic marker is a specific human leukocyte antigen that is closely associated with the disease. The presence of HLA B27 is a significant risk factor for developing AS, and while not all individuals with this antigen will develop the condition, the vast majority of patients diagnosed with AS test positive for it.

HLA B27 is part of the immune system and plays a crucial role in presenting peptides to T-cells, which are essential in immune response. The strong association of HLA B27 with Ankylosing Spondylitis provides insight into the autoimmune nature of the disease and aids in the diagnosis.

The other assays mentioned are not relevant to Ankylosing Spondylitis in the same context. For instance, CEA (carcinoembryonic antigen) is primarily used as a tumor marker, while the Philadelphia chromosome is associated with chronic myelogenous leukemia and the Ret proto-oncogene is linked to certain types of cancers and not autoimmune diseases like AS.