Healio
Healio

 Back to Ankylosing Spondylitis

AnkylosingSpondylitis_OG

Ankylosing Spondylitis

Laboratory Findings

Muhammad Asim Kahn, MD, FRCP, MACP; Nurullah Akkoç, MD 
Reviewed on July 15, 2024
email icon linkedin twitter facebook

Introduction

There is no specific “diagnostic” or pathognomonic laboratory marker for ankylosing spondylitis/axial spondyloarthritis (AS/axSpA), and diagnosis is based on clinical history, physical examination and imaging findings. Acute phase reactants such as elevated C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR) are often used as part of the laboratory workup of inflammatory rheumatic diseases. Elevated ESR and CRP are more commonly found in AS patients with peripheral arthritis than in those with only axial disease. Their clinical utility is relatively limited because of their suboptimal sensitivity and specificity; CRP is elevated in 50%-60% of the patients with AS and 30%-40% of the patients with non-radiographic axial spondyloarthritis (nr-axSpA) only. Thus, a normal value does not exclude the presence of axSpA. Measurement of CRP is preferred over ESR for clinical assessment and follow-up, and for calculating the Assessment of Spondyloarthritis…

Introduction

There is no specific “diagnostic” or pathognomonic laboratory marker for ankylosing spondylitis/axial spondyloarthritis (AS/axSpA), and diagnosis is based on clinical history, physical examination and imaging findings. Acute phase reactants such as elevated C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR) are often used as part of the laboratory workup of inflammatory rheumatic diseases. Elevated ESR and CRP are more commonly found in AS patients with peripheral arthritis than in those with only axial disease. Their clinical utility is relatively limited because of their suboptimal sensitivity and specificity; CRP is elevated in 50%-60% of the patients with AS and 30%-40% of the patients with non-radiographic axial spondyloarthritis (nr-axSpA) only. Thus, a normal value does not exclude the presence of axSpA. Measurement of CRP is preferred over ESR for clinical assessment and follow-up, and for calculating the Assessment of Spondyloarthritis International Society (ASAS)-endorsed disease activity score named Ankylosing Spondylitis Disease Activity Scores (ASDAS). Other much less useful acute phase responses include elevated ferritin, mild thrombocytosis and low albumin.

Mild to moderate elevation of serum immunoglobulin A concentration is also commonly present, although studies seeking IgA antibodies to a variety of organisms have not been helpful. There is no association with rheumatoid factor and antinuclear antibodies, and the synovial fluid and synovial biopsy do not show markedly distinctive features compared with other inflammatory arthritides. Stool examination may be of value in patients with gastrointestinal (GI) symptoms to look for concomitant inflammatory bowel disease (IBD). When reactive arthritis is suspected on clinical grounds, bacterial studies might provide helpful information in some instances.

A high prevalence of anti-CD74 antibodies specific for the human leukocyte antigen (HLA) class II-associated invariant chain peptide (CLIP) in patients with axSpA has been reported. However, subsequent studies have demonstrated its low specificity that resulted in a conclusion of a low diagnostic value of the test but it may have some utility as a diagnostic aid in population that have a low association of HLA-B27 with axSpA.

HLA-B27 Test

HLA-B*27 is a normal gene; its prevalence in the general population and the strength of its association with AS and related spondyloarthritis (SpA) markedly differ among the various ethnic and racial groups worldwide (Table 9-1). HLA-B27 as a genetic marker protein is associated with all of the conventional forms of SpA, and differentiation among these diseases is based on their respective clinical findings (Table 6-1). Moreover, many of these patients can lack this gene, especially in certain populations. For example, among patients with primary AS (unassociated with psoriasis or IBD) only about 50% of African American are HLA-B27+ as compared to approximately 90% of northern European patients.

Testing for this genetic marker is relatively inexpensive and does not need to be repeated (unless for technical/laboratory error). It is simply one of the means used to help in search for a diagnosis, and cannot be thought of as a routine, diagnostic, confirmatory, or screening test for AS/axSpA in patients presenting with back pain or arthritis. The appropriateness of HLA-B27 test in such a clinical situation will depend not only on patient-specific (patient’s ethnicity and race) but also on epidemiologic factors (pretest probability of the disease). This requires physicians understanding of probability reasoning (Bayesian analysis), a widely used method for the clinical interpretation of laboratory testing, as explained in detail elsewhere by the author, but is mostly not adhered to by physicians in clinical practice.

The prevalence of HLA-B27 in the general population (from which is derived its specificity) and the strength of its disease association (from which is derived its sensitivity) vary markedly among many ethnic and racial groups (Table 9-2). Bayes’ theorem states that the predictive value of a particular laboratory test depends on the sensitivity and specificity of that test, as well as the prevalence (or pre-test likelihood) of disease in the population (or the patient) being tested. If the clinical history and physical examination findings do not suggest AS/axSpA, testing for HLA-B27 will be inappropriate because a positive result would still not permit the diagnosis of AS to be made as it can be present in healthy subjects. This test has demonstrated clinical utility in recognizing AS/axSpA in certain clinical situations. For example, if the history and physical examination findings suggest AS but the radiographic findings do not permit this diagnosis to be made, the HLA-B27 test may allow the presumptive diagnosis of axSpA to be accepted or rejected with lesser uncertainty.

The presence or absence of HLA-B27 cannot establish or exclude the diagnosis of AS/axSpA because this test lacks 100% specificity and 100% sensitivity required to “rule in” or “rule out” associated disease. The test is most useful for diagnosing AS among Japanese population because of its strong association with this disease (85% sensitivity) and a <1% prevalence of HLA-B27 in the general population (>99% specificity), giving a positive likelihood ratio = >85 and of negative likelihood ratio = ~0.15). In comparison, a negative HLA-B27 test for AS in African Americans is clinically not very helpful in lowering the post-test probability of AS, but a positive result is even more helpful as an aid to diagnosis (positive likelihood ratio (16.7) in African Americans than in US whites (positive likelihood ratio 12.1) (Table 9-2).

HLA-B27 typing is most useful when performed in a clinical “toss-up” situation, i.e., ~50% pre-test probability (or in the range of 30% to 70%) in the presence of normal or equivocal sacroiliac imaging findings, where it can be used as a tie-breaker (Figure 9-1). Thus, the value of the HLA-B27 test as one of the means used to help in search for a diagnosis of AS/axSpA depends on the individual pretest probability of the disease and the patient’s ethnicity and race. This test may also help ophthalmologists to better identify a patient presenting with acute anterior uveitis, and to refer HLA-B27(+) patients for rheumatology consultation, especially those with associated musculoskeletal symptoms because up to 80% of them have or will develop AS or a related SpA.

Laboratory testing for HLA-B27 and CRP levels help in the ASAS classification criteria for axSpA. Although HLA-B27 typing of the general population can define those at higher risk for AS or related SpA, it is of very limited practical value for that purpose because no effective means of prevention is currently available and most (~95%) of the HLA-B27(+) individuals in the general population never develop AS or a related SpA. The risk of development of AS or any type of SpA even among HLA-B27(+) first-degree relatives of a HLA-B27(+) patient with AS is approximately 25% among patients of European descent, much higher than the 1% to 5% risk in the general public with HLA-B27. The differences between AS patients who possess HLA-B27 versus those who lack this gene are discussed in Etiology.

Polygenic risk scores (PRS) has recently been reported to perform better than HLA-B27 testing and other standard tests employed in AS including CRP measurement and MRI scanning (Figure 9-2). The authors have suggested that polygenic risk score (PRS) should be used to assist in diagnosing AS in patients with chronic back pain and are under 45 years of age and of European or East Asian descent. They have further proposed that PRS should be developed for use in specific ethnic groups in which they are to be applied.

Enlarge 
Enlarge 
Enlarge 
Enlarge  Figure 9-1: Probability Graph Depicting Diagnostic Value of HLA-B27 Testing in White and African-American Patients With AS. White patients with AS (A) sensitivity = 92%; specificity = 92%. African-American patients (B) sensitivity = 50%; specificity = 98%. Predictive values of positive and negative test results are shown for various pre-test probabilities. Solid curved lines indicate post-test probability of the disease, given a positive or a negative HLA-B27 test result. Magnitude of shift (vertical lines) from the 45º oblique dotted line depicts incremental gain expected from the HLA-B27 test in confirming or excluding AS, according to pre-test probabilities. For example, if HLA-B27 testing for AS is done in a patient in whom the physician considers the likelihood of disease to be 50:50 (pre-test probability is 50%) a positive test result would mean that there is a 92% probability that the person has the disease. Alternatively, a negative test result would decrease that probability to 8%. Source: Modified from Khan MA, Khan MK. Ann Intern Med. 1982;96(1):70-76.
Figure 9-1: Probability Graph Depicting Diagnostic Value of HLA-B27 Testing in White and African-American Patients With AS. White patients with AS (A) sensitivity = 92%; specificity = 92%. African-American patients (B) sensitivity = 50%; specificity = 98%. Predictive values of positive and negative test results are shown for various pre-test probabilities. Solid curved lines indicate post-test probability of the disease, given a positive or a negative HLA-B27 test result. Magnitude of shift (vertical lines) from the 45º oblique dotted line depicts incremental gain expected from the HLA-B27 test in confirming or excluding AS, according to pre-test probabilities. For example, if HLA-B27 testing for AS is done in a patient in whom the physician considers the likelihood of disease to be 50:50 (pre-test probability is 50%) a positive test result would mean that there is a 92% probability that the person has the disease. Alternatively, a negative test result would decrease that probability to 8%. Source: Modified from Khan MA, Khan MK. Ann Intern Med. 1982;96(1):70-76.
Enlarge  Figure 9-2: Receiver Operating Characteristic Curve Plot of Performance Polygenic Risk Scores. Purple line, area under the curve (AUC)=0.924); blue line, HLA-B27 (AUC=0.869); green line, PRS without MHC (AUC=0.782); and red line, genome-wide significant loci only (AUC=0.659). Source: Li Z, et al. Ann Rheum Dis. 2021. doi: 10.1136/annrheumdis-2020-219446.
Figure 9-2: Receiver Operating Characteristic Curve Plot of Performance Polygenic Risk Scores. Purple line, area under the curve (AUC)=0.924); blue line, HLA-B27 (AUC=0.869); green line, PRS without MHC (AUC=0.782); and red line, genome-wide significant loci only (AUC=0.659). Source: Li Z, et al. Ann Rheum Dis. 2021. doi: 10.1136/annrheumdis-2020-219446.
 

References

  • Kahn MA, Akkoç N. Ankylosing Spondylitis— Axial Spondyloarthritis, 3rd ed. Professional Communications Inc. 2023
  • Baraliakos X, Baerlecken N, Witte T, Heldmann F, Braun J. High prevalence of anti-CD74 antibodies specific for the HLA class II-associated invariant chain peptide (CLIP) in patients with axial spondyloarthritis. Ann Rheum Dis. 2014;73(6):1079-1082.
  • de Winter JJ, van de Sande MG, Baerlecken N, et al. Anti-CD74 antibodies have no diagnostic value in early axial spondyloarthritis: data from the spondyloarthritis caught early (SPACE) cohort. Arthritis Res Ther. 2018;20(1):38.
  • Elyan M, Khan MA. Diagnosing ankylosing spondylitis. J Rheumatol Suppl. 2006;78:12-23.
  • Khan A. A worldwide overview: the epidemiology of HLA-B27 and associated spondyloarthritides. The Spondyloarthrides. 1998;17.
  • Khan M, Sieper J. Reactive arthritis. In: Koopman W, Moreland L, eds. Arthritis and Allied Conditions. 15th ed. Philadelphia, PA: Lippincott Williams and Wilkins; 2004:1335-1355.
  • Khan M, van der Linden S, Villiger P, et al. Genetic influences on occurrence of axial spondyloarthritis (axSpA) in first-degree relatives during a prospective study lasting 35 years. Arthritis Rheumatol. 2020;72(suppl 10). Abstract.
  • Khan MA, Haroon M, Rosenbaum JT. Acute anterior uveitis and spondyloarthritis: more than meets the eye. Curr Rheumatol Rep. 2015;17(9):59.
  • Khan MA, Khan MK. Diagnostic value of HLA-B27 testing ankylosing spondylitis and Reiter’s syndrome. Ann Intern Med. 1982;96(1):70-76.
  • Khan MA. Clinical application of the HLA-B27 test in rheumatic diseases. A current perspective. Arch Intern Med. 1980;140(2):177-180.
  • Li Z, Wu X, Leo PJ, et al. Polygenic Risk Scores have high diagnostic capacity in ankylosing spondylitis. Ann Rheum Dis. 2021. doi:10.1136/annrheumdis-2020-219446.
  • Mackiewicz A, Khan MA, Reynolds TL, van der Linden S, Kushner I. Serum IgA, acute phase proteins, and glycosylation of alpha 1-acid glycoprotein in ankylosing spondylitis. Ann Rheum Dis. 1989;48(2):99-103.
  • Mease P, Khan MA, eds. Axial Spondyloarthritis, 1st ed. St. Louis, MO: Elsevier; 2020:1-294.
  • Rudwaleit M, Haibel H, Baraliakos X, et al. The early disease stage in axial spondylarthritis: results from the German Spondyloarthritis Inception Cohort. Arthritis Rheum. 2009;60(3):717-727.
  • Ziade NR, Mallak I, Merheb G, et al. Added Value of Anti-CD74 Autoantibodies in Axial SpondyloArthritis in a Population With Low HLA-B27 Prevalence. Front Immunol. 2019;10:574.
More in Ankylosing Spondylitis
 Previous Physical Findings
Next  Imaging