INFO - Clinical utility of common serum rheumatologic tests

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American Family Physician

15 Mar 2002

Clinical Utility of Common Serum Rheumatologic Tests

Excerpt:

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

Read the full article here:

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

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Not an MD