Clinical Utility of Common Serum Rheumatologic Tests

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American Family Physician

March 15, 2002

" Clinical Utility of Common Serum Rheumatologic Tests " :

" Rheumatoid Factor

The RF detected by standard laboratory testing is an IgM antibody

directed against the Fc (crystallizable fragment) portion of IgG.

Laboratory tests are capable of detecting other classes of rheumatoid

factors (e.g., IgG and IgA); however, these tests are not widely used

clinically.

A number of rheumatic and nonrheumatic conditions are associated with

positive RF tests (Table 1).3 RF is present in approximately 80 percent

of patients with rheumatoid arthritis.3 RF testing is also commonly

positive in patients with Sjögren's syndrome or cryoglobulinemia.

Nonrheumatic conditions frequently associated with the presence of RF

include bacterial endocarditis, tuberculosis, sarcoidosis and

malignancies. The prevalence of RF in healthy elderly patients may be as

high as 10 percent, although the titer is usually low (1:40 or lower).3

RF testing may be appropriate in patients suspected of having rheumatoid

arthritis.4 The test is most useful when there is a moderate level of

suspicion for rheumatoid arthritis.3 If clinical suspicion is low (i.e.,

absence of joint inflammation), RF testing is unlikely to be helpful

because of the high incidence of false-positive results in the general

population. Even when clinical suspicion is high, 20 percent of patients

with rheumatoid arthritis are seronegative.3 Furthermore, up to 40

percent of patients with rheumatoid arthritis may be seronegative early

in the course of the disease.3 RF testing in these circumstances may

influence the physician away from the true diagnosis.

In patients with rheumatoid arthritis, the RF titer generally correlates

with extra-articular manifestations and disease severity. RF testing may

have prognostic value in these patients. However, RF titers are not

helpful in following disease progression. Once a patient has a positive

RF result, repeating the test is of no value.

The specificity of RF for rheumatoid arthritis ranges from 80 to 95

percent, depending on the age and health of the population studied.5,6

The sensitivity of RF ranges from approximately 10 percent in patients

with polymyositis to more than 90 percent in those with Sjögren's

syndrome or cryoglobulinemia.5

RF testing is a useful screening tool when Sjögren's syndrome or

cryoglobulinemia is suspected. Serial RF measurements can be helpful in

patients with Sjögren's syndrome because the disappearance of RF may

herald the onset of lymphoma.7 "

http://www.aafp.org/afp/20020315/1073.html

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