Rheumatoid Factor Revisited
Bento Rui Mascarenhas
Despite improved understanding of mechanisms of disease, there is no gold standard for the diagnosis of autoimmune diseases. A clinician therefore needs to use a set of criteria for the diagnosis of an autoimmune disease. This article deals with the methods of detection of rheumatoid factor including more accurate methods like nephelometry, it’s interpretation and clinical signiﬁcance. A clinician should have this knowledge for the appropriate diagnosis and treatment of patients with arthritis.
Rheumatoid factor (RF) has been the most widely used test for a number of years in the evaluation of patients with arthritis. In recent years, an improved understanding of pathophysiology of rheumatoid arthritis has led to inclusion of the newer, anti-CCP antibody test in the revised criteria for diagnosis of the disease. However, rheumatoid factor has not yet been excluded. Therefore, RF still remains an important tool in the hands of a clinician, especially in resource limited settings, considering the higher cost of anti-CCP antibody testing. the abbreviation RA factor is a misnomer and it should always be referred to as Rheumatoid Factor or RF.
Autoantibodies are a characteristic feature of autoimmune rheumatic diseases, directly against highly speciﬁc targets. These originate from B cells which recognise self antigens as foreign. This immune dysregulation occurs in genetically predisposed individuals who are exposed to infection or other insults such as cell derived micro particles.
However, mere presence of autoantibodies is not synonymous with autoimmune diseases as these can be observed in otherwise healthy individuals. Autoantibodies in the presence of characteristic clinical features helps in the diagnosis of autoimmune disease. In the absence of a gold standard for the diagnosis of rheumatological diseases, they are best used as part of a diagnostic panel rather than as a marker indicating one particular disease.
Many autoimmune diseases are associated with a speciﬁc autoantibody at signiﬁcantly high titres compared to the level present in individuals without that particular disease. Thus, interpretation of a test report requires an understanding of the cut oﬀ value for a particular antibody. The upper limit of an antibody level which is present in not more than 5% of the normal population is ﬁxed as the cut oﬀ level of an autoantibody for that speciﬁc disease.
Rheumatoid factors (RF) are autoantibodies directed against Fc portion of human immunoglobulin and may be of IgM, IgG and IgA isotypes. (Figure 1)
Methods of Detection :
(1) Tests to detect RF by passive haemagglutination were initially described by Rose and Waaler (1940). This detects anti-Rabbit IgG and is expressed as a ratio (e.g. >1:16 is positive).
(2) RF latex agglutination test detects anti-Human IgG and results are expressed as IU/ml (>20 IU is positive). The reciprocal of highest dilution of the serum which is able to agglutinate IgG coated latex particles is expressed as titre. The titre can be converted to International units ( I U ) by multiplying the titre with the conversion factor provided in the kit. For e.g. if the highest dilution is 1/20, the titre is 20 and if the conversion factor is 8, then the RF level is calculated as 20×8=160 IU. There is a reference preparation (WHO 1066) and most methods are calibrated against this standard.
(3) ELISA detects class speciﬁc IgG, IgA and IgM antibodies and these are not routinely required. IgM RF is the one currently measured in commercial laboratories.
(4) Turbidimetry and Nephelometry : These methods measure the rate of formation of immune complexes in solution. These are more widely used because of speed, accuracy and automation. These can be used to measure RF, CRP, ASO, C3 and C4 levels. (Figure 2 & 3)
Turbidimetry measures a decrease in intensity of light as it passes through a solution of particles. Whereas, nephelometry measures light energy scattered towards a detector – not in direct path of transmitted light but placed at 60⁰-70⁰ angle from incident light. The amount of light scattered is proportional to concentration of antigen or antibody in solution. Since the amount of light scattered is greater than transmitted light in a turbid suspension, nephelometry is more sensitive than turbidimetry with a detection limit of 10 µg/ml, while with turbidimetry it is 20-30 µg/ml. This is superior to other techniques as it is able to detect changes in absolute levels at earlier stages and has a low inter assay co-eﬃcient of variance.
RF has a sensitivity of 70-80% at a titre more than 20 IU, but speciﬁcity is poor. Therefore, a positive or negative result neither conﬁrms nor excludes rheumatoid arthritis. RF is also found in 1-5% of healthy population and higher in elderly people (3-25%) without rheumatic disease. When present in this population, RF is found in low-moderate titre (1:40-1:160). There are a number of conditions in which RF may be falsely positive. (Table 1 & 2)
There is a huge variation between methods used for detection and quantiﬁcation of autoantibodies. Do not assume that results will be interchangeable between laboratories. Hence, interpretation of results must be made in the background of clinical presentation :
- If RF positive along with characteristic clinical presentation, the positive predictive value is about 80%. This includes patients with symmetrical polyarthralgia or polyarthritis involving small joints, prominent morning stiﬀness or sicca symptoms.
- In early inﬂammatory arthritis, the presence of RF predicts a future diagnosis of rheumatoid arthritis with a positive odds ratio of approximately 30. These patients require aggressive therapy, because presence of RF early in the course of disease is associated with erosion and extra-articular features.
- If RF is negative consistently in a patient presenting with typical clinical features of rheumatoid arthritis, then other serological markers (anti-CCP antibody) are to be tested for early intervention. RF may be negative in 20-50% of patients with rheumatoid arthritis.
- If RF is positive in patients without typical features, other causes of RF positivity are to be ruled out.
Clinical Case Study :
A 50-year-old lady presents with asymmetrical, inﬂammatory polyarthritis since 2 years. There is no history of low back pain or systemic symptoms. She gives a history of treatment for psoriasis since 1 year and there are no lesions presently. There is clinical synovitis of bilateral metacarpophalangeal joints, right wrist, both knees and left ankle joint. There is sausage shaped swelling of 2nd/3rd proximal interphalangeal joints of right hand and distal interphalangeal joint of left 4th toe(dactylitis) as shown in (Figure 4). Her Rheumatoid factor is positive 414 IU .
Therefore, this RF is falsely positive in psoriatic arthritis and the patient may be inappropriately diagnosed and treated as rheumatoid arthritis. A proper history of asymmetrical arthritis with or without low back pain and uveitis should be sought. A careful examination for psoriatic skin lesions (especially hidden areas), nail changes and sacroilitis is essential. Clinicians need to be aware of the fact that arthritis can precede the onset of skin lesions in about 15-20% of patients with psoriatic arthritis by years.
CLINICAL SIGNIFICANCE OF RHEUMATOID FACTOR :
(1) There is a correlation between higher RF concent rat ions and systemic symptoms, vasculitis and more severe forms of the disease.
(2) Despite low sensitivity, RF has value as a screening test for polyarthralgias. Anti-CCP testing helps because of its speciﬁcity of 90%.
(3) Higher titres of RF have higher positive predictive value for Rheumatoid arthritis.
(4) RF is positive in 75-95% of patients with Sjogren’s syndrome, which may or may not be associated with RA. Therefore, these patients are invariably labelled and treated as Rheumatoid arthritis unless high index of clinical suspicion.
IMPORTANT LEARNING POINTS :
(1) Rheumatoid factor shows no correlation with disease activity. Therefore, need not be repeated every time the patient is followed up.
(2) The clinician should be aware of the laboratory method used for testing and always ask for titres. Laboratories should be accredited and participate in external quality assurance schemes.
(3) If laboratory results are completely inconsistent with the clinical ﬁndings, request anti-CCP test.
- Rao URK. Manual of Rheumatology. 4th edition. Indian Rheumatology Association. 2014.
- Sheldon J. Laboratory testing in autoimmune rheumatic diseases. Best Practice and Research Clinical Rheumatology. 2004. Vol.18, No.3. 249-269.
- Roberts-Thomson PJ, McEvoy R, Langhans T, Bradley J. Routine quantiﬁcation of Rheumatoid factor by rate nephelometry. Ann Rheum Dis. June 1985, 44(6):379-383.
- Symposium on Laboratory Diagnosis of Autoimmune Diseases, IRACON December 2011.
- Shmerling RH, Wener MH, Romain P. Origin and utility of measurement of rheumatoid factors, www.uptodate.com Jan-Feb 2016.