Monday, February 18, 2019

Labs in SLE

Laboratory testing — We obtain the following routine laboratory tests, which may provide diagnostically useful information:

●Complete blood count and differential may reveal leukopenia, mild anemia, and/or thrombocytopenia

●Elevated serum creatinine may be suggestive of renal dysfunction

●Urinalysis with urine sediment may reveal hematuria, pyuria, proteinuria, and/or cellular casts

In addition to the routine laboratories described above, we perform the following laboratory tests which support the diagnosis of SLE if abnormal:

●ANA

●Antiphospholipid antibodies (lupus anticoagulant [LA], IgG and IgM anticardiolipin [aCL] antibodies; and IgG and IgM anti-beta2-glycoprotein [GP] I)

●C3 and C4 or CH50 complement levels

●Erythrocyte sedimentation rate (ESR) and/or C-reactive protein (CRP) levels

●Urine protein-to-creatinine ratio

The ANA test is positive in virtually all patients with SLE at some time in the course of their disease . If the ANA is positive, one should test for other specific antibodies such as dsDNA, anti-Sm, Ro/SSA, La/SSB, and U1 ribonucleoprotein (RNP). In some labs, a positive ANA test by indirect immunofluorescence will automatically result in testing for such additional antinuclear antibodies that are often present in patients SLE.

●Anti-dsDNA and anti-Sm antibodies are highly specific for SLE, but anti-Sm antibodies lack sensitivity . Anti-dsDNA and anti-Sm antibodies are seen in approximately 70 and 30 percent of patients with SLE, respectively.

●Anti-Ro/SSA and anti-La/SSB antibodies are present in approximately 30 and 20 percent of patients with SLE, respectively; however, both antibodies are more commonly associated with Sjögren's syndrome.

●Anti-U1 RNP antibodies are observed in approximately 25 percent of patients with SLE, but they also occur in patients with other conditions and high levels are almost always present in patients with mixed connective tissue disease (MCTD).

●Antiribosomal P protein antibodies have a high specificity for SLE, but have low sensitivity for SLE. They also lack specificity for involvement of a particular organ system or disease manifestation.

If the initial ANA test is negative, but the clinical suspicion of SLE is high, then additional antibody testing may still be appropriate. This is partly related to the differences in the sensitivity and specificity among the methods used to detect ANA. A more detailed discussion on the techniques used to detect ANA is presented separately.

Fun fact : Anti-Sm antibody was actually named after a patient with Lupus, Mr. Smith.

Bhopalwala. H

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