Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (723 page)

   Multiplex immunoassay (MIA) tests have been recently developed for use in clinical laboratory. They utilize individually identifiable, fluorescence microspheres (beads), each coupled with a different antigen or antigen mixture to test for multiple antibodies simultaneously in the same tube. This multiplex ANA screen is intended for qualitative screening of specific ANAs, the quantitative detection of dsDNA antibodies, and semiquantitative detection of 10 separate antibody assays (chromatin, ribosomal-P, SSA, SSB, Sm, SmRNP, RNP [ribonucleoprotein], Scl-70 [topoisomerase I], Jo-1, and centromere-B). This ANA by MIA screen detects the presence of clinically relevant circulating autoantibodies in serum. These assays are specific compared to IFA, and they are not as sensitive as IFA, because it is not looking at 100–150 possible antigens in the Hep-2 cells, rather specifically looking at 11 specific targeted antibodies. These assays have typical sensitivities of 66–94% for SLE, 94% for Sjögren, 68% for systemic sclerosis, and 48% for PM-DM. They are specific compared to IFA for detecting specific targeted connective tissue disorders. In persons with no connective disease, the specificity of MIA ranged from 77 to 91%, and in apparently healthy individuals, it is at 93%.

   Disorders associated with a positive ANA titer include chronic infectious diseases, such as mononucleosis, hepatitis C infection, subacute bacterial endocarditis, TB, and HIV, and some lymphoproliferative diseases.

   The presence of ANAs is rarely associated with malignancy, with the exception of dermatomyositis, in which both may be present. ANAs have also been identified in up to 50% of patients taking certain drugs; however, most of these patients do not develop drug-induced lupus. Drugs that may cause positive results include carbamazepine, chlorpromazine, ethosuximide, hydralazine, isoniazid, mephenytoin, methyldopa, penicillins, phenytoin, primidone, procainamide, and quinidine.

   
Antibodies to double-stranded DNA (dsDNA)

   Moderate to high titers of antibodies directed against dsDNA are very specific (97%) for SLE, making them very useful for diagnosis. Anti-dsDNA have also been found at low frequency (<5%), and usually in low titer and with low avidity, in patients with RA, Sjögren syndrome, scleroderma, Raynaud phenomenon, MCTD, discoid lupus, myositis, uveitis, juvenile arthritis, antiphospholipid syndrome, Grave disease, Alzheimer disease, and autoimmune hepatitis.

   Titers of anti-dsDNA antibodies often fluctuate with disease activity and are, therefore, useful in many patients for following the course of SLE.

   There is a well-recognized association of high titers of IgG anti-dsDNA titers, especially for high avidity antibodies, with active GN; there also appear to be highly enriched amounts of anti-dsDNA antibodies in the glomerular deposits of immune complexes found in patients with lupus nephritis. These observations have led many investigators to believe that anti-dsDNA antibodies are of primary importance in the pathogenesis of lupus nephritis.

   Anti-dsDNA antibodies have also been reported in patients receiving minocycline, etanercept, infliximab, and penicillamine.

   An increased frequency of these antibodies has also been noted in some otherwise normal individuals, particularly first-degree relatives of patients with lupus and some laboratory workers.

   
Antibodies to chromatin

   Chromatin refers to the complex of histones and DNA. Assaying for the presence of antichromatin (antinucleosome) antibodies may be more clinically relevant than testing for individual antihistone antibodies. Antichromatin antibodies are present in 69% of those with SLE but in 10% or less of patients with Sjögren syndrome, scleroderma, or antiphospholipid syndrome. Among those with SLE, the prevalence of antichromatin antibodies is twofold higher in those with renal disease (58% vs. 29%).