rheumatoid factors ( rfs ) , a class of immunoglobulins ( igs ) that have different isotypes and affinities , were first detected more than 70 years ago , but there is still much to discover about the mechanisms underlying their production , physiological role , and pathological effects . waaler described an antibody directed against serum gamma - globulins that promoted the agglutination of sheep red blood cells sensitised by subagglutinating doses of rabbit antibodies in 1940 , although it had actually been previously found in patients with liver cirrhosis and chronic bronchitis by kurt meyer in 1922 . in 1948 , rose described these antibodies in patients with rheumatoid arthritis ( ra ) , and in 1952 they were finally christened rfs because of their association with ra . however , although they owe their name to their first detection in ra patients , rfs are found in patients with other autoimmune and nonautoimmune diseases , as well as - in healthy subjects . the aim of this review is to describe the clinical applications of testing for rfs . classic agglutination techniques were initially used because of the ability of igms to induce agglutination . the first rf detection assay was based on the fact that rf agglutinates sheep red blood cells sensitised with rabbit iggs ( i.e. , the classic waaler - rose test ) [ 2 , 3 ] , and this was followed by the development of other igg carriers such as bentonite [ 5 , 6 ] and latex particles [ 7 , 8 ] . automated techniques such as nephelometry and enzyme - linked immunosorbent assays gradually replaced the other semiquantitative methods because of their simplicity and greater reproducibility [ 912 ] . multiplexed immunoassaying is an emerging high - throughput technique for the quantitative detection of multiple analytes from a single biological sample . although they have yet to be standardised and validated , multiplexed immunoassays can reduce analytical time and enhance accuracy . however , it is known that rfs can interfere with a number of laboratory immunoassays and lead to false positive results : for example , in patients with high rf levels , the analysis of vancomycin can be compromised if serum rather than plasma samples are used [ 14 , 15 ] . rfs can also interfere with other laboratory tests , including those designed to detect anticardiolipin antibodies ( especially if igm levels are in the low positive range ) , anti-2gpi antibodies , anti - hcv antibodies , antirubella antibodies , thyroid assays [ 20 , 21 ] , and tests for carbohydrate antigen 199   and various cytokines . as shown in table 1 , rfs can be detected in patients with many nonrheumatic conditions . infections and chronic diseases may be characterised by the presence of serum rfs , but unlike those detected in ra patients , the rfs produced during infections are usually transient and not detrimental . given the ability of rfs to increase the clearance of immune complexes and the fact that rf - producing b cells may behave as antigen - presenting cells ( apcs ) and aid the immune response against the infectious antigens , it is likely that the net impact of rf production during infections is protective for the host [ 24 , 26 ] . these natural rfs are generally low - affinity , polyreactive igm antibodies produced by cd5-positive b cells [ 27 , 28 ] , and coexistence of rf - positive b cells and nonautoimmune igg antigen in healthy subjects suggests the existence of tolerance mechanisms . rfs can be found in 4050% of patients with hcv infection , but their frequency can reach 76% . their production is probably due to chronic stimulation of the immune system by hcv , and , as hcv infection is highly prevalent in various countries ( 1.53% in southern europe ) and represents the first cause of increased serum rfs , hcv antibodies should be sought in all subjects with increased rf levels [ 29 , 30 ] ( figure 1 ) . rf positivity has also been reported in the healthy population [ 3133 ] , and up to 4% of young caucasians may be rf positive , with a similar distribution between the two genders . it is thought that genetic and environmental factors are responsible for the worldwide variability in distribution of rfs : for example , their highest prevalence ( up to 30% ) has been observed in north american indians tribes [ 3436 ] . the rfs found in healthy subjects are different from those present in ra patients as their titres are low / moderate and they are likely to be produced by cd5-expressing b cells as low - affinity , poly - reactive igms without any signs of maturation affinity . the transient production of low - affinity igm rfs may be induced by polyclonal b cell activators such as bacterial lipopolysaccharides and epstein - barr virus [ 28 , 37 ] , but it has been shown that high rf titres in healthy subjects predict the development of ra . furthermore , igm rfs are sometimes observed in healthy elderly people , which suggests that they may be a consequence of the age - related immune deregulation ( figure 1 ) [ 39 , 40 ] . rfs are frequently detected in patients with systemic autoimmune diseases , such as systemic lupus erythematosus , mixed connective tissue disease , polymyositis , and dermatomyositis ( table 1 ) [ 24 , 25 ] . patients with sjogren 's syndrome ( ss )   and those with type ii and iii mixed cryoglobulinemia ( usually hcv related )   have the highest rf titres . about 60% of the patients with primary ss are rf positive , with males having higher iga rf levels than females . it is also thought that the disease - related transformation of activated rf - positive b cell clones is involved in the pathogenesis of the lymphoproliferative disorders that develop in about 5% of ss patients . most ss patients have high titres of polyclonal rfs , whereas monoclonal rfs can be detected in patients with type ii mixed cryoglobulinemia and , to a lesser extent , in ss patients with lymphoproliferative disorders [ 24 , 34 ] . although rfs can be detected in patients with other connective tissue diseases , rf isotypes are helpful in the management of ra patients from the time of diagnosis until deciding on the choice of therapeutic strategy ( figures 1 and 2 ) [ 44 , 45 ] . rf testing in ra patients has a sensitivity of 60% to 90% and a specificity of 85% [ 46 , 47 ] . a number of hypotheses have been postulated in order to explain the possible key role of rfs in ra , including their capacity to increase the elimination of immune complexes by macrophages , the improved cytotoxicity of antiviral antibodies , and the increased elimination of parasites . it has also been suggested that rfs potentiate the presentation of antigens to t cells by means of the dendritic cell uptake of immune complexes with exogenous antigens and by means of rf b cells , which seem to be more efficient apcs than other b cells   ( figure 3 ) . finally , it is possible that the rapid secretion of large amounts of low - affinity rfs prevents the activation of higher - affinity rf b cells and additional b cells [ 5153 ] .   defining rfs as anti - igg or anti - gamma - globulins is inaccurate because it restricts rf reactivity to the igg fc fragment . igm rfs are the most frequently detected isotype , but igg , iga , ige , and igd rfs can also be observed . it has been shown that three rf isotypes ( igm , iga , and igg ) are detected in up to 52% of ra patients moreover , the presence of iga and igg rf isotypes in absence of igm - rf is more prevalent in patients with connective tissue diseases than in ra patients , whereas an increase in both igm and iga rfs is almost exclusively observed in patients with ra [ 55 , 56 ] . it has long been recognised that rfs play a pivotal role in the differential diagnosis of polyarthritis because they make it possible to identify ra patients . for this reason , rf testing has been one of the classification criteria for ra since 1987   and , although many years have passed since their identification , their crucial role in classifying ra has been confirmed by the updated criteria .   however , in order to increase the specificity of the latest ra classification criteria , anti - cyclic citrullinated protein / peptide antibody ( acpa ) testing has been added . a meta - analysis   has shown that the pooled sensitivities of acpa and rf are similar , but acpa positivity is more specific for ra than igm rf , igg rf , or iga rf positivity   and more specific for early ra than igm rf . on the other hand , sensitivity is reduced because positivity for both acpa and rf is a more stringent criterion than positivity for either alone ; combining acpa and rf positivity is more permissive in terms of sensitivity because the antibodies complement each other , especially for early ra [ 6366 ] . furthermore , although the cut - off value of each commercial kit is slightly different , it has been suggested that the best acpa cut - off value should be 40  u / ml , which leads to a positive likelihood ratio of 5.49 and a negative likelihood ratio of 0.50 [ 46 , 67 ] . it has also been shown that rfs are useful in predicting the development of ra , as the detection of igm , iga , and igg rfs may predate its onset by years [ 38 , 68 ] , and it has been reported that their appearance in serum is sequential before diagnosis : first igm rf , then iga rf , and finally igg rf [ 57 , 69 ] . the detection of igm rfs is also helpful as a prognostic index , and some studies have shown that immunosuppressive treatment can decrease serum rf levels . however , the clinical usefulness of rfs in monitoring disease activity   and treatment response is limited . it has been shown that a progressive decrease in the rf levels parallels the decrease of clinical activity in patients treated with traditional disease modifying antirheumatic drugs   or biologic agents such as infliximab [ 7375 ] , etanercept , adalimumab , rituximab [ 78 , 79 ] , and abatacept or tocilizumab [ 80 , 81 ] . there are conflicting published data concerning the potential role of rfs in predicting responses to antitumor necrosis factor alpha ( tnf- ) : some studies have found that rf positivity before therapy is insufficient to predict a response [ 8285 ] , whereas others have found that it predicts a negative response [ 86 , 87 ] . in particular , it has been reported that high pretreatment levels of iga rf are associated with a poor clinical response to tnf- inhibitors . high serum levels of rf are predictors of more severe disease forms and b cell - depleting therapy can have a beneficial effect : rf - positive ra patients have a better response to rituximab than those who are rf negative [ 8992 ] . it has been demonstrated that low - affinity rfs appear to be key player in immune responses to many infectious organisms , and high - affinity rfs indicate more severe and persistent disease in patients with ra . rfs are probably the result of the immune response to inflammation ( depending on genetic background ) and may have regulatory effects on ig production by controlling b cell activation .