Hi Kieran! Welcome to AxSpA.net.
Axial spondyloarthritis is a clinical diagnosis. This means there is no 1 specific test result that gives a definitive diagnosis. There are physical exams, symptoms, imaging tests, and labs that should be performed during the course of establishing a diagnosis.
A diagnosis is obtained by analyzing and interpreting the data and results of your labs, imaging, and exams and comparing those with the criteria of the disease.
I believe the "blood screening" you may be referring to is blood taken to find the presence of the HLA-B27 protein. This is a specific type of protein that is known to contribute to dysfunction in a person's immune system, especially AS, but not in all cases. Even if not present, there is a chance you may still have an autoimmune disease. This test, too, is not exhaustive in diagnosing AxSpA.
The article below may be able to shed some light on the subject. Please contact us again if you have others questions or comments.
Thanks for your great question!
https://ankylosingspondylitis.net/diagnosis/
Rebecca (moderator for AxSpA.net)