Investigational biologic markers in the diagnosis and assessment of rheumatoid arthritis

Investigational biologic markers in the diagnosis and assessment of rheumatoid arthritis

Authors
Peter C Taylor, MA, PhD, FRCP
Ravinder N Maini, BA, MB BChir, FRCP, FMedSci, FRS

Section Editor
James R O'Dell, MD

Deputy Editor
Paul L Romain, MD

All topics are updated as new evidence becomes available and our peer review process is complete.

Literature review current through: May 2016. &#124 This topic last updated: Jun 03, 2016.

INTRODUCTION — Patients with rheumatoid arthritis (RA) follow a variable disease course with regard to outcome measures such as functional status or radiological assessment of joint damage. Early identification of patients with RA and, in particular, of those likely to assume a more rapidly destructive form of disease is important because of the possible benefit from early, aggressive intervention with disease-modifying agents. This realization has prompted the investigation and measurement of numerous biologic “markers” in blood and joint fluids that may serve as indicators of prognosis and the response to therapy. Although some of the markers under consideration are accessible in routine practice, many are in the stage of experimental evaluation and require access to specialized technology and customized reagents.
CLINICALLY USEFUL MARKERS — Among the many biologic markers that have been assessed for usefulness in estimating disease activity and prognosis of rheumatoid arthritis (RA), only a few have found a role in clinical practice. The main clinically useful biologic markers in patients with RA include rheumatoid factors (RF), anti-cyclic citrullinated peptide (anti-CCP) antibodies, erythrocyte sedimentation rate (ESR), and C-reactive protein (CRP). These tests and their clinical use in diagnosis and assessment of disease outcome are discussed in detail elsewhere. (See "Biologic markers in the diagnosis and assessment of rheumatoid arthritis".)
INVESTIGATIONAL MARKERS — Potential biologic markers that remain investigational can be considered in four categories:
●Immunologic (or serologic) abnormalities
●Genetic factors, such as human leukocyte antigen (HLA) class II
●The acute phase response elicited by hepatocytes as part of the inflammatory process, other than C-reactive protein (CRP) or those assessed indirectly by erythrocyte sedimentation rate (ESR)
●Macromolecules specific for joint-associated tissues that are released into the circulation or excreted in the urine as part of degenerative and reparative processes
Although not measured clinically, the pattern of cytokine activation may be important in the pathogenesis of rheumatoid arthritis (RA). This topic is discussed elsewhere. (See "Role of cytokines in rheumatic diseases" and "Pathogenesis of rheumatoid arthritis".)
Immunologic abnormalities — Several immunologic abnormalities, particularly autoantibodies, are associated with the presence of rheumatoid arthritis.
Autoantibodies
Antiperinuclear factor and antikeratin antibodies — Characterization of the antigenic targets recognized by antikeratin and antiperinuclear antibodies led to the recognition of citrullinated peptides as important antigenic targets for antibodies present in the sera of patients with RA. Commercially available assays for the detection of anti-citrullinated peptide antibodies (ACPA), including antibodies to cyclic citrullinated peptides (anti-CCP), are widely used clinically in the evaluation of RA. (See "Biologic markers in the diagnosis and assessment of rheumatoid arthritis", section on 'Anti-citrullinated peptide antibodies'.)
Antikeratin antibodies (AKA) were originally identified in 1979 in patients with RA [1]. A related antibody, antiperinuclear factor antibody (APF), is also associated with RA [2]. Study of APF and AKA led to the appreciation that both types of antibodies recognize epidermal filaggrin, an intermediate filament-associated protein involved in cornification of the epidermis [3,4]. A major determinant of antibody binding to filaggrin is the presence of citrulline, formed by a post-translational modification of arginine. Citrullination is catalyzed by peptidylarginine deiminase [5]. Antibodies against this antigen had higher specificity for RA than did rheumatoid factor (RF). However, assays for these antibodies were technically demanding and never widely available.
Identification of these and other post-translationally modified peptides and proteins containing the amino acid citrulline led to the development of the more convenient and standardized assays for antibodies to citrullinated proteins, including enzyme-linked immunosorbent assays (ELISA) for anti-CCP antibodies. (See "Biologic markers in the diagnosis and assessment of rheumatoid arthritis", section on 'Anti-citrullinated peptide antibodies'.)
Testing for antibodies to other citrullinated proteins has been an active area of investigation. As examples, the presence of antibodies to citrullinated fibrinogen, to citrullinated vimentin (anti-Sa) (see 'Anti-Sa antibodies' below), to citrullinated synthetic type I or type II collagen telopeptides, and to alpha-enolase may have sensitivity and specificity similar to that of anti-CCP testing [6-13]. A commercial assay for anti-mutated citrullinated vimentin is also available. (See "Biologic markers in the diagnosis and assessment of rheumatoid arthritis", section on 'Anti-MCV antibodies'.)
The following discussion of APF and AKA is, therefore, of mainly historical interest.
The sensitivity and specificity of APF and AKA vary depending on the laboratory means of detection.
●APF detected by means of indirect immunofluorescence using buccal epithelium has a reported specificity for RA of 73 to 99 percent [2,14,15] and a sensitivity of 49 to 87 percent [2,14,16].
●The reported specificity of AKA, detected by the use of rat esophagus as a substrate [1], generally exceeds 90 percent with a sensitivity between 40 and 60 percent [14,17].
●An immunoblotting method for the detection of AKA, using three protein antigens extracted from rat esophagus epithelium and separated by nondenaturing polyacrylamide gel electrophoresis, found that AKA had a sensitivity of 43 to 50 percent and a specificity of 99 percent for RA [18]. The intensity of labeling of the three rat esophagus protein antigens, and that of human epidermal filaggrin when RA sera are tested by immunoblotting, correlates with the APF titers of the sera [3].
Despite the lack of widespread availability of tests to detect AKA and APF (and anti-filaggrin antibodies), identifying their presence in a given patient may have important implications.
●AKA have been reported in RF-negative RA, although the prevalence is only one-third that in RF-positive cases [14]. Similarly, APF has been reported in cases of RA that are repeatedly RF-negative [1].
●The prognostic significance of AKA and APF has been studied in three cohorts of early RA with follow-ups of three and eight years, respectively [19-21]. All patients with detectable AKA developed at least some erosions by the end of the eight-year follow-up [21]. One cohort with recent-onset RA was followed by the same investigators; the initial presence of AKA was associated with active and treatment-resistant disease at up to three years but did not predict radiologic progression [22]. Others have reported similar findings [19]. Furthermore, among patients with chronic erosive RA, APF and AKA do not define any subgroup with a particularly aggressive course.
●Similar to RF, the presence of AKA, APF, or antifilaggrin antibodies may antedate clinical RA [23-25]. In addition, detection of either AKA or APF in a healthy individual with a positive sensitized sheep red blood cell agglutination test further increases the relative risk of developing RF-positive RA by a factor of five [24]. However, a significant number of antibody-positive patients do not develop disease. As an example, in the study just quoted, 12 of 70 RF positive patients who did not develop RA tested positive for either AKA, APF, or both antibodies [24].
●A negative AKA may help exclude a diagnosis of RA in patients who are RF-positive and present with symptoms suggestive of RA but who have other disorders. In one study, for example, the prevalence of AKA was 8 percent among patients with hepatitis C virus (HCV) infection, RF positivity, and arthralgia or arthritis, compared with 61 percent for those diagnosed with RA who were RF-positive and were not infected with HCV [26].
Positive RF, AKA, or APF tests prior to clinical disease may also indicate that initiation of disease antedates symptoms by many years. This possibility has profound implications for epidemiologic studies seeking an initiating cause of RA.
Anti-Sa antibodies — Anti-Sa antibodies, similar to a number of other autoantibodies linked to RA, recognize a citrulline-containing epitope located on vimentin, an intermediate filament that is widely expressed on mesenchymal cells and macrophages that is not normally citrullinated [27]. Antibodies to mutated citrullinated vimentin (anti-MCV) may be associated with more severe disease than anti-CCP antibodies and may be comparable to anti-CCP in predicting radiographic progression [9,28]. A commercial assay for antibodies to MCV is available, and the information below is presented for historical perspective. (See "Biologic markers in the diagnosis and assessment of rheumatoid arthritis", section on 'Anti-MCV antibodies'.)
The following observations suggested that the presence of anti-Sa antibodies may be an accurate diagnostic and predictive test for RA:
●In a review of 489 patients with inflammatory joint disease, of whom 154 had RA, anti-Sa antibodies were 68 percent sensitive and 79 percent specific for RA [29]. Anti-Sa antibodies also helped predict which patients would eventually develop late severe radiologic damage.
●A second study compared the use of anti-Sa and anti-CCP antibodies in an outpatient clinic population comprised of patients with RA, other connective tissue diseases, and spondyloarthritides [30]. Anti-Sa antibodies were much less sensitive (44 versus 72 percent) with a similar specificity (96 versus 94 percent). Anti-Sa and anti-CCP antibodies were discordant in 47 of 87 patients with RA. Thus, assays for anti-Sa antibodies were proposed as being useful when anti-CCP antibodies are negative but RA is still suspected.
Other autoantibodies — A number of other autoantibodies have also been the subject of investigation:
●Anti-p68 (BiP) antibodies – A protein that serves as a chaperone in the endoplasmic reticulum and binds to immunoglobulin heavy chains is a p68 autoantigen named BiP. Increased T-cell proliferative responses against BiP have been noted in patients with RA, and antibodies directed against the protein may be a marker for the disease. As an example, in one study of 400 patients with RA, 200 patients with other rheumatic diseases, and 150 healthy donors, antibodies with anti-BiP specificity had a 68 and 96 percent sensitivity and specificity for RA, respectively [31].
●Anti-RA33 antibodies – Immunoblot analysis using soluble nuclear extracts from HeLa cells demonstrated the presence of an autoantibody to a 33 kD antigen (anti RA33) in 4 of 14 patients with RA in a report from Austria; no positive tests were noted in patients with ankylosing spondylitis or psoriatic arthropathy [32].

The RA33 antibody is directed against a functional component of the spliceosome and is also found in systemic lupus erythematosus (SLE) and mixed connective tissue disease (MCTD); however, the pattern on immunoblot is distinguishable from RA by the simultaneous presence of Sm bands (BB', D) and U1 RNP (A, C), respectively [33]. (See "Antibodies to double-stranded (ds)DNA, Sm, and U1 RNP" and "Anti-U1 RNP antibodies in mixed connective tissue disease".)

Anti-RA33 antibodies are associated with prominent arthritis in patients with MCTD and SLE [34], suggesting a relationship of this antibody to autoimmune inflammatory joint disease in general. Furthermore, it has been postulated that anti-RA33 antibodies are an early and stable marker of RA, since all seven patients in the Austrian study who had initially unclassified anti-RA33 positive arthritis went on to develop RA within three years [32]. However, the presence of anti-RA33 antibodies is not associated with an increased risk of progressive radiographic joint damage [19].
●ANCA – Patients with RA may have antineutrophil cytoplasmic antibodies, usually atypical (non-MPO) perinuclear ANCA (p-ANCA). Nonvasculitic aspects of RA activity, severity, and chronicity have not consistently correlated with ANCA status. One report found that patients with RA who were p-ANCA positive had an increased rate of radiographic progression of joint damage [35]. However, until this is confirmed, it is our opinion that there is little clinical utility for ANCA testing in patients with RA in whom the presence of an ANCA-associated systemic vasculitis is not suspected on clinical grounds. (See "Clinical spectrum of antineutrophil cytoplasmic antibodies", section on 'Nonvasculitic rheumatic disorders'.)
●Gal 0 glycoforms – RA is associated with a marked increase in IgG lacking galactose (designated as Gal 0 glycoforms) and terminating in N-acetyl glucosamine in the Fc region [36]. Gal 0 levels correlate with disease severity in RA and revert to normal during the course of pregnancy-induced remissions [37]. Agalactosyl immunoglobulins (Gal 0 IgG) activate complement by means of a mannose binding protein dependent pathway in vitro [38]. More rapid radiographic progression of RA has been associated with low serum levels of mannose-binding lectin, which is determined genetically [39,40]. Furthermore, the finding of mannose binding protein and enrichment of Gal 0 IgG in RA synovial fluids has prompted the suggestion that these events are of pathologic significance in the joint destruction of RA.

Gal 0 IgG and RF were measured at presentation and at a two-year follow-up in a prospective study of 60 British patients with a history of synovitis of less than one year's duration [41]. Over the study period, 39 patients developed RA as defined by the 1987 American College of Rheumatology classification criteria for RA (see "Clinical manifestations of rheumatoid arthritis" and "Diagnosis and differential diagnosis of rheumatoid arthritis", section on 'Classification criteria'). The initial Gal 0 IgG correctly predicted the development of RA, or otherwise, in 47 of the 60 patients. The combination of Gal 0 IgG and RF status gave a test with 90 percent sensitivity, 95 percent specificity, and 94 percent predictive value.
●Antibodies to alpha-enolase – Among 255 patients with early arthritis, elevated serum levels of antibodies to an enzyme in the glycolytic pathway, alpha-enolase, were found in 25 percent of those who were later determined to have RA [42]. One-half of those with RA and anti-enolase antibodies did not have serum RF or antifilaggrin antibodies. Among 40 patients whose arthritis remained undifferentiated after one year of observation, 10 percent had antibodies to alpha-enolase.
●Antibodies to glucose phosphate isomerase (GPI) – Antibodies to the ubiquitous enzyme, glucose phosphate isomerase, which have been implicated as pathogenically important in an animal model of inflammatory arthritis, may be associated with the presence of extraarticular manifestations of RA (see "Pathogenesis of rheumatoid arthritis", section on 'Glucose-6-phosphate isomerase'). These autoantibodies are more often present in those patients with RA complicated by rheumatoid vasculitis and Felty syndrome [43].
●Antibodies to ferritin – Antibodies to the heavy chain of ferritin may be associated with RA. This association in a predominantly Caucasian population is suggested by the finding in one study of a higher prevalence of such antibodies in those with established RA and early RA (16 and 19 percent, respectively) than in healthy blood donors, osteoarthritic subjects, or those with SLE (2.7, 2.1, and 2.1 percent, respectively) [44].
●Antibodies to PADI4 – It is known that the enzyme peptidyl-arginine deiminase 4 (PADI4) converts arginine to citrulline and that genetic variants of this enzyme are associated with RA. The presence of antibodies to PADI4 in patients with RA and in controls was examined in Chinese patients [45]. Levels of antibodies to PAD4 were assessed in 109 patients with RA, in 232 patients with related rheumatic diseases, and in 106 healthy controls. Antibodies to PAD4 were observed in 45 percent of patients with RA and in 0 to 13 percent of controls.
Cell surface expression of CD40 ligand — The CD40 ligand (CD154) is expressed on activated T cells and underlies many interactions between these and other hematopoietic cells. Enhanced surface expression of the CD40 ligand on circulating T cells is associated with more active disease in RA [46].
Complement activation — Markers of complement activation, possibly due to immune complexes present in synovium or plasma, have been proposed as a possible indicator of RA disease activity. As an example, the concentration of covalently linked C1q-C4 complement components was correlated with disease activity in 84 patients with RA [47].
Genetic factors — The risk of developing RA is associated with carriage of particular HLA alleles. A patient's HLA DR type and other genetic factors may also play a role in determining the severity of disease.
Shared epitope — RA is positively associated with certain HLA DR alleles (particularly HLA DR4) encoding a conserved amino acid sequence in the third hypervariable region of the DRb1 chain. A number of studies have suggested the potential usefulness of HLA typing in predicting disease severity and as a guide to early aggressive therapy. This area of investigation is discussed in detail separately. (See "HLA and other susceptibility genes in rheumatoid arthritis".)
Matrix metalloproteinase genotype — Matrix metalloproteinases can degrade collagen and can contribute to cartilage and bone destruction in RA (see "Pathogenesis of rheumatoid arthritis"). Carriage of a polymorphism in the promoter region of the gene for matrix metalloproteinase 3 (MMP-3) may be associated with more severe disease. This was illustrated in one study of 103 patients with early RA [48]. Homozygous carriage of a particular polymorphism in the promoter region of the MMP-3 gene(6A/6A) was associated with the presence of more severe joint damage at baseline and, after four years of follow-up, was associated with more progression of joint erosions and joint space narrowing than carriage of one or more alleles of a different type (5A).
Interleukin-10 promoter genotype — An association may exist between an interleukin-10 (IL-10) promoter polymorphism and the severity of, though not the susceptibility to, RA. This was illustrated in a study in 283 Dutch patients who were prospectively followed for two years [49]. There was no difference between the patients with RA and healthy controls in the prevalence of two IL-10 promoter polymorphisms (-2849 G and A), and there was no significant difference in the baseline radiographic severity of their joint disease. However, among the 152 patients with at least one allele associated with an increased production of IL-10, there was a significantly higher mean joint damage score by the end of the study than among the 31 patients without such an allele.
Sulphoxidation status — The inherited ability to oxidize the sulphur compound S-carboxy-methyl-L-cysteine varies widely among individuals. The prevalence of individuals with poor sulphoxidation status is increased in patients with established RA [50]. Furthermore, one study suggested that a defective sulphoxidation status correlates strongly with persistent clinical disease in early RA. In this report, the S-oxidation capacity was evaluated in 54 patients with recent onset symmetrical polyarthritis, and the clinical course was monitored at for four years. At four years, 74 percent of patients with a diagnosis of RA were poor S-oxidizers compared with 31 percent of those who were asymptomatic [50].
Acute phase response — Markers of the acute phase response, such as CRP, the ESR, and IL-6, have been evaluated as potential markers of disease activity in RA. (See "Acute phase reactants" and "Biologic markers in the diagnosis and assessment of rheumatoid arthritis".)
Interleukin-6 — IL-6 has a major stimulatory effect on hepatic synthesis of acute-phase proteins [51] and is known to play a regulatory role in platelet production [52]. Furthermore, there is evidence to suggest that IL-6 has an etiopathologic role in the anemia of chronic disease [53]. (See "Anemia of chronic disease/inflammation".)
Inflamed synovium is thought to be the principal source of plasma IL-6 in RA, since IL-6 is often detected in high concentration in the synovial fluid. Thus, it was postulated that plasma IL-6 concentrations might reflect joint inflammation better than acute-phase protein levels. However, in one study of 51 patients with early RA, there was no relationship between radiologic progression and time-integrated values of plasma IL-6 concentration even though there was a significant correlation between IL-6 and the acute phase response [54]. This finding contrasts with the relationship between radiologic findings and time-integrated values of CRP and ESR discussed above.
Tissue-specific markers — A number of biochemical markers of joint damage have been described in RA. These molecules may be synthetic or degradative, their presence in body fluids arising as a consequence of metabolism of the tissue of origin. They are predominantly derived from a single tissue such as cartilage, bone, or synovium and can be detected principally by immunoassay of joint fluid, serum, or urine. Appropriate assays are not routinely available, and joint fluids are not readily available (apart from knee joints), in comparison with serum or urine. There is, as yet, limited information regarding the usefulness of tissue-specific markers as measures of disease activity or response to therapy in RA. However, some markers may be of prognostic value.
Synovium-specific markers — The synovium is thought to be the dominant source of serum hyaluronan, a marker that is strikingly elevated in the serum of patients with RA [55,56]. In vitro studies demonstrate that synovial lining cells of rheumatoid joints produce detectable amounts of hyaluronan, while lining cells from normal joints do not [57]. Despite a short half-life of 15 minutes or less, serum hyaluronan concentrations correlate with disease activity [56], and at least one prospective study has suggested that, in early RA, serum hyaluronan may reflect ongoing joint destruction and may even predict subsequent joint damage [58]. However, elevated serum levels of hyaluronan may be nonspecific since they may vary with physical activity independent of the degree of synovitis [59].
Other markers that may be predominantly released from the synovium are MMP-1 and MMP-3, enzymes that fragment matrix collagen. Elevated levels of MMP-3 [60] and/or MMP-1 [61] may correlate with increased radiographic joint damage. Elevated serum levels of immunoglobulin soluble receptor (Fc gamma RIIIa) in RA are thought to be due to synovial macrophagesand/or NK cells [62].
An isoform of the 14-3-3 family of chaperone proteins, 14-3-3eta, which has a molecular weight of approximately 28 kDa, is present in the sera and synovial fluid of a majority of patients with early and established RA [63,64]. In synovial fluid, the levels of 14-3-3eta are more than fivefold greater than in matched serum specimens [63]. Serum 14-3-3eta may be helpful diagnostically in RA, as it may have similar sensitivity and specificity to RF and ACPA in distinguishing patients with RA from osteoarthritis, other autoimmune disorders, and healthy controls [64]. Serum levels of 14-3-3eta in inflammatory arthritis correlate with levels of MMP-1 and MMP-3, and the protein possesses ligand activity, preferentially activating cells of the innate immune system to activate proinflammatory cytokines, including IL-1, IL-6, and tumor necrosis factor (TNF) alpha, as well as factors involved in joint injury such as MMP-9 and RANK ligand [65]. Nonetheless, serum levels of 14-3-3eta do not correlate with levels of acute phase markers or composite disease activity scores, although its presence is associated with more severe disease [64].
Cartilage-specific markers — Markers of cartilage metabolism may have some prognostic value in patients with RA. In early RA, for example, high serum levels of cartilage oligomeric matrix protein (COMP), a member of the thrombospondin protein family, predict severe disease characterized by subsequent large- and small-joint destruction [66,67]. In one study, increased serum concentrations of COMP were found in all patients who developed rapid hip joint destruction [67].
The aggrecan content of synovial fluid in knee joints is also reported to be predictive of knee and hip joint destruction [68]. The chondroitin sulphate-rich region of aggrecan is most abundantly detected in synovial fluids recovered from joints with little radiologic evidence of destruction, whereas the hyaluronan binding region of core protein is released in more severely damaged joints [69].
The same study that measured serum level of COMP in patients with RA (see above) also measured serum levels of a putative marker of cartilage aggrecan synthesis, epitope 846, located on the chondroitin sulphate rich area of the aggrecan molecule. The epitope 846 levels were found to be elevated only in a group of patients with slow joint destruction, as compared with a group matched for age, gender, and disease duration but with more destructive joint disease [67]. These data indicate the presence of cartilage reparative processes in the group with a more benign course and suggest that elevated 846 epitope is indicative of a more favorable prognosis.
Measurement of crosslinked c-terminal peptides from type II collagen (CTX-II) in urine may provide some prognostic information. A correlation between the excretion of these peptides and radiographic progression up to five years was noted in a prospective study of 110 patients with early RA [70,71]. Similarly, urinary excretion of a peptide derived from the helical portion of type II collagen (HELIX-II) also correlates with radiographic progression and is independent of other variables, including baseline CRP levels, joint damage, and urinary CTX-II excretion [72]. When studied in 89 patients with early RA, patients with increased levels of both HELIX-II and CTX-II had the highest risk of radiographic progression compared with those without an elevation of either of these markers (OR 17.5, 95% CI 3.1-99).
The combination of cartilage and synovial biomarkers may have a better ability to predict radiographic progression than either alone. This was illustrated in a study of 118 patients in which several biomarkers were measured in blood samples and in which radiographs were obtained at baseline and after two years of treatment with synthetic disease-modifying antirheumatic drugs (DMARDs) but not with biologic agents; multivariable analysis indicated that the combinations of serum biomarkers that included both MMP-3 and CTX-II levels were better than any individual biomarker in predicting radiographic progression [73].
Bone-specific markers — As with cartilage, several bone-specific markers are available and may have a useful purpose in patients with RA.
●Bone sialoprotein is an osteoblast-derived protein preferentially expressed in juxtaarticular bone. Bone sialoprotein levels in synovial fluid correlate with knee joint destruction in both RA and osteoarthritis [74]. By contrast, bone sialoprotein levels are elevated in RA serum without a correlation between concentration and joint destruction.
●Bone degradation, assessed by detection of pyridinoline cross-links in urine, correlates with disease activity in RA and diminishes after treatment with pulsed glucocorticoids and DMARDs [75].
●Immunoassays are available for measurement of cross-linked carboxyterminal telopeptides of type I collagen (ICTP), a larger serum marker for bone collagen degradation. A three-year prospective study of 66 patients with early RA found that 51 percent initially had elevated levels of serum ICTP compared with healthy controls. Throughout the follow-up, serum ICTP levels correlated with inflammatory parameters and, from the first year on, with the radiologic changes assessed annually. Initial ICTP levels correlated better than the other variables of disease activity with the subsequent erosive progression of joints, suggesting that its measurement may serve as a prognostic marker for joint damage in early RA [76]; this observation was independently confirmed in a group of 110 patients with early RA [70]. Subsequent studies found that ICTP levels in synovial fluid correlated better with prognosis than did serum levels [77] and that higher ICTP levels after six months of treatment with a combination of antirheumatic drugs was predictive of increased radiographic progression [78].
●The initial ratio of serum levels of an inhibitor of osteoclast differentiation (osteoprotegerin) and a stimulatory protein (Receptor Activator of Nuclear factor Kappa B ligand [RANKL]) may be a predictor of joint destruction in patients with early RA [79].
Vascular markers — Serum vascular endothelial factor (VEGF) concentrations are elevated in RA patients when compared with healthy controls and with patients with osteoarthritis [80-82]. Furthermore, in patients with early RA, there is a significant correlation between serum VEGF levels at presentation and radiographically assessed joint damage over the subsequent year [81].
Plasma proteins — In a proteomic approach, 163 plasma proteins were evaluated in 44 patients with RA to determine which were associated with disease activity [83]. Plasma proteins whose concentrations were correlated with disease activity included IL-6, oncostatin M, IL-2, macrophage colony-stimulating factor (M-CSF), TNF receptor superfamily member 9, C-C motif chemokine 23 (CCL23), transforming growth factor (TGF) alpha, and chemokine C-X-C motif ligand 13 (CXCL13).
Multi-protein biomarker algorithms — The combined use of multiple markers may provide advantages over the use of single markers for predicting disease activity and progression. In order to develop a multi-biomarker disease activity (MBDA) test for RA, 130 candidate biomarkers were tested in feasibility studies and 25 were selected for algorithm training. Multi-biomarker statistical models outperformed individual biomarkers at estimating disease activity assessed by the disease activity score (DAS) using a 28 joint count and C-reactive protein level (DAS28-CRP). The final MBDA algorithm used 12 biomarkers to generate an MBDA score between 1 and 100 [84]. Emerging data indicate that a MBDA test may provide a useful adjunct to clinical assessment to identify progression-free remission and assess subclinical disease. In a study of 163 patients with RA in the Leiden Early Arthritis Cohort, at a total of 271 visits, patients with a high MBDA score were 2.3 times more likely (95% CI 1.1-3.7) to have joint damage progression during the subsequent year [85].
SUMMARY AND RECOMMENDATIONS — Early identification of patients with rheumatoid arthritis (RA) and, in particular, of those likely to assume a more rapidly destructive form of disease is important because of the possible benefit from early, aggressive intervention with disease-modifying agents. This realization has prompted the investigation and measurement of numerous biologic “markers” in blood and joint fluids that may serve as indicators of prognosis and the response to therapy. Although some of the markers under consideration are accessible in routine practice, many are in the stage of experimental evaluation and require access to specialized technology and customized reagents. (See 'Introduction' above.)
●The main clinically useful biologic markers in patients with RA include rheumatoid factors, anti-cyclic citrullinated peptide (anti-CCP) antibodies, erythrocyte sedimentation rate (ESR), and C-reactive protein (CRP). (See 'Clinically useful markers' above.)
●Biologic markers that remain investigational can be considered in four categories (see 'Investigational markers' above):
•Immunologic (or serologic) abnormalities
•Genetic factors, such as human leukocyte antigen (HLA) class II
•The acute-phase response elicited by hepatocytes as part of the inflammatory process, other than CRP or those assessed indirectly by ESR
•Macromolecules specific for joint-associated tissues that are released into the circulation or excreted in the urine as part of degenerative and reparative processes
●Several immunologic abnormalities, particularly autoantibodies, are associated with the presence of rheumatoid arthritis. Such autoantibodies include antiperinuclear factor, antikeratin antibodies, and anti-Sa, which have all been found to be directed against citrullinated peptides. These antibodies, which are now mainly of historical interest since the development of commercial tests for anti-citrullinated peptide/protein antibodies, may have both diagnostic and prognostic significance. (See 'Autoantibodies' above.)
●Other markers of interest include levels of CD154, measures of complement activation, and other autoantibodies, including (see 'Autoantibodies' above and 'Cell surface expression of CD40 ligand' above and 'Complement activation' above):
•Anti-p68 (BiP) antibodies, which target a protein that serves as a chaperone in the endoplasmic reticulum and binds to immunoglobulin heavy
•Anti-RA33 antibodies, directed against a functional component of the spliceosome and also found in systemic lupus and mixed connective tissue disease
•Antineutrophil cytoplasmic antibody (ANCA), usually atypical (non-myeloperoxidase) p-ANCA
•Agalactosyl immunoglobulins (Gal 0 IgG), which activate complement by means of a mannose binding protein dependent pathway in vitro
•Antibodies to alpha-enolase
•Antibodies to glucose phosphate isomerase
•Antibodies to ferritin
•Antibodies to peptidyl-arginine deiminase 4 (PADI4)
●Genetic factors that may influence the severity of RA include the shared epitope, matrix metalloproteinase genotype, interleukin (IL)-10 promoter genotype, and sulphoxidation status. (See 'Genetic factors' above and "Biologic markers in the diagnosis and assessment of rheumatoid arthritis", section on 'Genetic factors'.)
●Markers of the acute phase response, such as CRP, the ESR, and IL-6, have been evaluated as markers of disease activity in RA. (See 'Acute phase response' above.)
●A number of experimental biochemical markers of joint damage have been described in RA. These molecules may be synthetic or degradative, their presence in body fluids arising as a consequence of metabolism of the tissue of origin. They are predominantly derived from a single tissue such as cartilage, bone, or synovium and can be detected principally by immunoassay of joint fluid, serum, or urine. (See 'Tissue-specific markers' above.)