Ankylosing spondylitis (AS) is a chronic inflammatory rheumatic disease that primarily affects the sacroiliac joints and spine, potentially leading to progressive structural damage and spinal fusion if not diagnosed early. Reliable detection and analysis—combining HLA-B27 genotyping, inflammatory biomarker measurement, and advanced imaging assessment—are essential for early diagnosis, differential diagnosis, and long-term disease monitoring. This article provides a comprehensive technical overview of ankylosing spondylitis testing and analysis, covering sample types, testing items, methodologies, applicable standards, and clinical applications.
Technical Overview
Ankylosing spondylitis (AS) is a chronic, progressive inflammatory disease belonging to the family of axial spondyloarthritides (axSpA). It predominantly affects the sacroiliac joints and the axial skeleton, causing inflammatory back pain, enthesitis, and, in advanced stages, syndesmophyte formation and vertebral ankylosis. The disease typically begins before the age of 40 to 45 years and shows a strong genetic association with the human leukocyte antigen B27 (HLA-B27), which is present in approximately 90% to 95% of affected patients in most populations.
Because early-stage symptoms often overlap with mechanical low back pain, the average diagnostic delay has historically ranged from 5 to 10 years. A definitive diagnostic workup therefore integrates three complementary pillars: clinical evaluation based on established criteria such as the 1984 Modified New York Criteria and the 2009 ASAS (Assessment of SpondyloArthritis International Society) classification criteria for axial spondyloarthritis; laboratory analysis of genetic and inflammatory markers; and imaging assessment of the sacroiliac joints and spine using radiography, computed tomography (CT), and magnetic resonance imaging (MRI).
All laboratory procedures described in this article should be performed in medical laboratories accredited to ISO 15189 (Medical laboratories — Requirements for quality and competence), with method validation, internal quality control, and participation in accredited external quality assessment programs ensuring analytical accuracy and clinical reliability.
Testing Samples
EDTA Anticoagulated Whole Blood
EDTA (ethylenediaminetetraacetic acid) anticoagulated venous blood is the primary specimen for HLA-B27 antigen detection by flow cytometry, complete blood count analysis, and extraction of genomic DNA for molecular genotyping. Samples should be collected in EDTA tubes, stored at 2 °C to 8 °C, and analyzed ideally within 24 to 48 hours to preserve lymphocyte viability and membrane antigen integrity.
Serum Samples
Serum separated from clotted venous blood is used for quantitative measurement of inflammatory markers such as C-reactive protein (CRP) and high-sensitivity CRP, serum amyloid A (SAA), and for exclusion testing including rheumatoid factor (RF), anti-cyclic citrullinated peptide (anti-CCP) antibodies, and antinuclear antibodies (ANA). After centrifugation, serum should be analyzed promptly or stored at 2 °C to 8 °C for short-term use, or at −20 °C or below for longer storage.
Sodium Citrate Anticoagulated Whole Blood
Erythrocyte sedimentation rate (ESR) determination by the Westergren method requires whole blood collected in sodium citrate anticoagulant at a precise 4:1 blood-to-anticoagulant ratio. The test should be performed within 4 hours of collection when stored at room temperature, in accordance with CLSI and ICSH (International Council for Standardization in Haematology) recommendations.
Genomic DNA
For PCR-based HLA-B27 genotyping and subtyping, genomic DNA is extracted from EDTA whole blood or buccal swab specimens. Extracted DNA should meet defined quality criteria, typically a concentration of 10 to 50 ng/µL and an A260/A280 absorbance ratio of 1.8 to 2.0, and should be stored at −20 °C or below to prevent degradation.
Imaging Studies
Radiographs of the sacroiliac joints and spine, MRI examinations (including STIR and T1-weighted sequences), and CT scans constitute the imaging specimens for structural and active lesion assessment. Standardized projection techniques and sequence protocols are essential for reproducible grading and interpretation.
Testing Items
The ankylosing spondylitis testing and analysis portfolio typically encompasses the following items:
- HLA-B27 antigen detection by flow cytometry and HLA-B27 gene genotyping by molecular methods, including subtyping of clinically relevant alleles such as B*27:02, B*27:04, and B*27:05
- Erythrocyte sedimentation rate (ESR) measurement by the Westergren method
- C-reactive protein (CRP) and high-sensitivity CRP (hs-CRP) quantification by immunoturbidimetry or immunonephelometry
- Serum amyloid A (SAA) and inflammatory cytokines such as interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-α) for disease activity assessment
- Complete blood count to identify anemia of chronic disease and reactive thrombocytosis
- Exclusion panels: rheumatoid factor (RF), anti-CCP antibodies, and antinuclear antibodies (ANA) to differentiate AS from rheumatoid arthritis and other connective tissue diseases
- Pre-biologic safety screening: tuberculin skin test or interferon-gamma release assay (IGRA), hepatitis B serology, and baseline liver and renal function panels prior to anti-TNF or IL-17 inhibitor therapy
- Radiographic grading of sacroiliitis according to the Modified New York Criteria (Grade 0 to Grade 4)
- MRI evaluation of active inflammatory lesions, including bone marrow edema and osteitis of the sacroiliac joints, according to the ASAS positive MRI definition
- Disease activity and functional scoring support, including ASDAS-CRP/ASDAS-ESR calculation and BASDAI/BASFI-related laboratory inputs
Testing Methods
HLA-B27 Detection by Flow Cytometry
Flow cytometric analysis is the most widely used screening method for HLA-B27. Lymphocytes in EDTA whole blood are stained with fluorochrome-labeled monoclonal antibodies specific for HLA-B27, together with a lymphocyte gate marker such as CD45 or CD3. Fluorescence intensity is analyzed on a calibrated flow cytometer, and results are reported qualitatively as positive or negative based on validated cutoff values. The method offers high sensitivity and specificity, rapid turnaround, and low sample volume requirements, and should be validated in accordance with ISO 15189 and relevant CLSI immunophenotyping guidelines.
Molecular HLA-B27 Genotyping
For confirmatory or research purposes, polymerase chain reaction with sequence-specific primers (PCR-SSP), PCR with sequence-specific oligonucleotide probes (PCR-SSO), real-time PCR with allele-specific hydrolysis probes, or sequencing-based typing (SBT) can be performed on extracted genomic DNA. These techniques detect HLA-B27 at the nucleic acid level and permit allele-level subtyping, which is valuable for ethnicity-specific association studies and for resolving ambiguous antigen-negative but genotype-positive cases.
Inflammatory Marker Measurement
ESR is measured using the Westergren method, the internationally recognized reference technique, with automated sedimentation analyzers acceptable provided they are validated against the manual reference method. CRP and hs-CRP are quantified by immunoturbidimetric or immunonephelometric assays calibrated against certified reference materials. SAA and cytokines such as IL-6 are measured by immunoassay platforms including ELISA and chemiluminescent immunoassay. Elevated ESR and CRP support the presence of systemic inflammation, contribute to ASDAS disease activity scoring, and serve as objective parameters for monitoring therapeutic response.
Autoantibody and Exclusion Testing
RF is typically quantified by immunoturbidimetry, anti-CCP antibodies by ELISA or chemiluminescent immunoassay, and ANA by indirect immunofluorescence on HEp-2 cells. Negative results in a patient with inflammatory back pain support the exclusion of rheumatoid arthritis and systemic lupus erythematosus, reinforcing the clinical suspicion of axial spondyloarthritis.
Radiographic Analysis
Conventional radiographs of the sacroiliac joints, obtained with standardized oblique projections, are graded according to the Modified New York Criteria: Grade 0 (normal), Grade 1 (suspicious changes), Grade 2 (minimal abnormality with small localized erosion or sclerosis), Grade 3 (unequivocal abnormality with erosions, sclerosis, widening or narrowing of the joint space, or partial ankylosis), and Grade 4 (total ankylosis). Definite radiographic sacroiliitis (Grade 2 bilaterally or Grade 3 to 4 unilaterally) combined with clinical criteria confirms classic AS.
MRI and Advanced Imaging Analysis
MRI using short tau inversion recovery (STIR) or fat-suppressed T2-weighted sequences can detect active sacroiliitis—bone marrow edema and osteitis—months to years before structural changes appear on radiographs. The ASAS definition of a positive MRI requires the presence of bone marrow edema lesions at more than one location within a single sacroiliac joint. CT may be employed for detailed characterization of erosions and subchondral sclerosis when radiographic findings are equivocal. Standardized interpretation should be performed by qualified radiologists using validated scoring systems such as the Spondyloarthritis Research Consortium of Canada (SPARCC) MRI index.
Applications
Ankylosing spondylitis testing and analysis serves multiple clinical and research purposes:
- Early diagnosis of axial spondyloarthritis in patients presenting with chronic inflammatory back pain, reducing diagnostic delay and enabling timely intervention
- Differential diagnosis from rheumatoid arthritis, mechanical back pain, degenerative disc disease, infection, and malignancy through combined laboratory exclusion panels and imaging
- Disease activity monitoring and treatment response evaluation using serial ESR, CRP, and ASDAS-CRP measurements during non-steroidal anti-inflammatory drug, anti-TNF, or IL-17 inhibitor therapy
- Pre-treatment safety screening for latent tuberculosis, hepatitis B infection, and organ dysfunction prior to initiation of biologic agents
- Genetic risk assessment and family counseling for first-degree relatives of HLA-B27-positive AS patients
- Prognostic evaluation through longitudinal imaging assessment of structural progression, syndesmophyte formation, and spinal mobility correlation
- Clinical research and epidemiological studies on HLA-B27 allele distribution, susceptibility genes such as ERAP1 and IL23R, and biomarker discovery
FAQ
Does a positive HLA-B27 result mean I definitely have ankylosing spondylitis?
No. Although approximately 90% to 95% of AS patients are HLA-B27 positive, a substantial proportion of the healthy general population also carries the antigen without ever developing the disease. HLA-B27 is a susceptibility marker, not a diagnostic test; a positive result must always be interpreted together with clinical symptoms and imaging findings.
Can AS be excluded if HLA-B27 is negative?
A negative result substantially reduces the probability of AS but does not absolutely exclude it, as roughly 5% to 10% of patients with confirmed AS are HLA-B27 negative. Patients with suggestive inflammatory back pain and imaging evidence of sacroiliitis should undergo full diagnostic evaluation regardless of the HLA-B27 status.
Why are ESR and CRP both measured if they show the same inflammation?
ESR and CRP reflect different biological processes and have complementary kinetics. CRP responds rapidly to inflammatory change, whereas ESR is influenced by immunoglobulins and fibrinogen over a longer time course. Combined measurement improves sensitivity for detecting active inflammation and provides more robust inputs for ASDAS-based disease activity scoring.
Can blood tests alone diagnose ankylosing spondylitis?
No. There is no single blood test that can definitively diagnose AS. Diagnosis requires the integration of clinical criteria, HLA-B27 and inflammatory marker results, and imaging evidence of sacroiliitis. Notably, up to 40% of AS patients have normal ESR and CRP values, underscoring the essential role of MRI in early disease detection.
How early can MRI detect sacroiliitis compared with X-rays?
MRI can visualize active inflammatory lesions such as bone marrow edema months to years before structural damage becomes visible on conventional radiographs. In patients with suspected early axial spondyloarthritis and normal radiographs, MRI according to the ASAS definition is the imaging method of choice.
How often should laboratory monitoring be performed?
For patients receiving biologic therapy, inflammatory markers, complete blood counts, and liver and renal function tests are generally monitored every 3 to 6 months, or more frequently as clinically indicated. Tuberculosis and hepatitis B screening should be repeated annually in endemic regions or per local clinical guidelines for patients on anti-TNF agents.
Is genetic testing recommended for family members of AS patients?
Because first-degree relatives of AS patients have an elevated risk of developing spondyloarthritis, HLA-B27 testing may be considered for relatives who present with suggestive symptoms. Testing of asymptomatic relatives should be accompanied by professional genetic counseling, since a positive result indicates elevated susceptibility rather than certainty of disease development.
