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Current Perspectives on Rheumatic Laboratory Tests

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Current Perspectives on Rheumatic Laboratory Tests Current Perspectives on Rheumatic Laboratory Tests Bernard Ng, MBBS, MMed; and Qurat Ul-Ain Kamili, MD Clinicians face a considerable dilemma when deciding which tests to utilize for the diagnosis and management of rheumatic diseases. These authors describe some of the most commonly used rheumatic tests and offer their recommendations as to when they should be ordered. espite advances in rheumatic ANTINUCLEAR ANTIBODY diseases because ANAs are present in therapeutics over the past de- (ANA) TESTING a variety of conditions2 (Table 1). cade, older tests continue to The lupus erythematosus (LE) cell In identifying SLE, the negative Dbe used for diagnostic pur- test—first described in 1948 by he- predictive value of the IIF-ANA test poses. Most rheumatic laboratory matologists Malcolm Hargraves and is estimated to be > 95%, but its posi- tests initially were described over 50 Robert Morton at the Mayo Clinic— tive predictive value (PPV) is only years ago and are based on the de- led to the discovery of ANAs.1 The 57%,4 which is even lower when the tection of autoantibodies. Although researchers observed that the LE cell test is ordered inappropriately. In 1 autoantibodies are associated with phenomenon occurred only in the study, the PPV of IIF-ANA testing rheumatic diseases, their pathoge- presence of what are now known as was only 29% for connective-tissue netic role remains unclear. In ad- antihistone antibodies, which are part diseases because the test was being dition, they have been detected in of the ANA family. ordered inappropriately for large individuals with unrelated disorders, The ANA test used most com- numbers of noninflammatory condi- such as infections and malignan- monly is indirect immunofluores- tions, including fibromyalgia and lo- cies, in the absence of any systemic cence (IIF) using human epithelial or calized soft-tissue rheumatism.5 autoimmune disorder. So, while rodent liver cells as substrates (with Although there are case reports autoantibody tests provide useful human epithelial cells being the more of “ANA-negative” SLE, it remains information, they require careful in- sensitive of the 2). With more than unclear whether these represent terpretation in conjunction with de- 30 nuclear antigens known to be as- a subgroup of SLE or whether they tailed history taking and physical sociated with autoimmune disorders, are technical artifacts.6 Defining examination. traditional enzyme-linked immuno- positivity at a higher titer makes In this article, we briefly review sorbent assay (ELISA), which detects the IIF-ANA test more specific but the most commonly used rheumatic a single antigen epitope, is less effi- less sensitive: At ANA titers of 1:80, tests and the historical context in cient than IIF in testing for ANAs. 1:160, and 1:320, the proportion of which they emerged. We discuss the IIF-ANA tests are highly sensitive normal patients testing positive is clinical importance of each in the for diagnosing systemic lupus erythe- 13.3%, 5%, and 3.3%, respectively.2 diagnosis and prognosis of various matosus (SLE) at the screening titer The different types of ANAs rheumatic disorders and make rec- of 1:40 (sensitivity > 97%).2 Specific- are defined by their target antigen: ommendations for their use in clini- ity is sacrificed, however, because up double-stranded (ds) DNA; single- cal practice. to one-third of the general population stranded DNA; nuclear histones; nu- may test positive for ANAs at that cleoproteins; and such RNA-protein Dr. Ng is a staff allergist and rheumatologist in level, and positivity increases with complexes as ribonucleoprotein the Medical Care Line as well as a physician in- age.2,3 Although the IIF-ANA test is (RNP), Smith, Scl-70, SSA, and SSB. vestigator in the Health Services Research and Development Center of Excellence at the Michael an excellent tool for detecting SLE In binding to nuclear antigens, ANAs E. DeBakey VA Medical Center in Houston, Texas. when there is a high degree of clinical produce different staining patterns on Dr. Kamili is an internal medicine resident in the suspicion for the condition, it is not a IIF, such as homogenous, speckled, Department of Internal Medicine at Baylor College of Medicine (BCM) in Houston, Texas. In addition, useful screening tool and should not nucleolar, and centromeric. These Dr. Ng is an assistant professor at BCM. be used to rule out other rheumatic patterns, however, are not sufficiently APRIL 2011 • FEDERAL PRACTITIONER • 11 CURRENT PERSPECTIVES ON RHEUMATIC LABORATORY TESTS sensitive or specific to diagnose rheu- matic disorders; for that, more spe- cific tests are required. ANTI-dsDNA ANTIBODY Autoantibodies to DNA were associ- ated with SLE in the mid 1960s7 and with nephritis shortly thereafter.8 The accurate measurement of this anti- body, however, was impeded by the lack of a good substrate for IIF. In 1975, Aarden and colleagues reported that the dsDNA in the kinetoplast of Crithidia luciliae could be used as a substrate for determining anti-dsDNA antibodies.9 The ELISA technique for detecting immunoglobulin (Ig) G an- tibodies to dsDNA is more sensitive but less specific than using C luciliae immunofluorescence (CLIF).10 The sensitivity and specificity of the CLIF test for anti-dsDNA in SLE range from 61% to 79% and 73% to 95%, respectively.10-12 The PPV for SLE is 50% with ELISA and 94% with CLIF,12 which is an important consid- eration when evaluating a patient’s laboratory studies for SLE, especially if the patient belongs to a population with a low SLE prevalence. Anti-dsDNA testing is a useful complement to positive IIF-ANA re- sults in patients with suspected SLE, but, because of its lower sensitivity, the test should not be ordered for pa- using phosphate-buffered saline ex- Of patients with SLE, 25% to 47% tients with negative IIF-ANA results. tract of calf thymus. The Smith anti- have anti-RNP antibodies and 5% to Anti-dsDNA tests correlate modestly gen consists of small nuclear RNPs. 30% have anti-Smith antibodies,15 with SLE disease activity and renal Anti-RNP antibodies react against though the method used for mea- involvement. Increasing titers may proteins involved in the splicing of surement affects both the sensitivity precede an SLE flare, but studies ad- heterogeneous nuclear RNA to mes- and specificity of these tests.16 Many dressing this observation are not con- senger RNA. Anti-Smith and anti- laboratories use ELISA because it can clusive. RNP antibodies typically are grouped be performed quickly and easily, but together because their antigen targets the early reports of sensitivity and ANTI-SMITH AND ANTI-RNP are found on the spliceosomes and specificity were based on immunopre- ANTIBODIES often coexist. Patients with anti-RNP cipitation techniques, such as coun- The anti-Smith antibody is named antibodies eventually develop anti- tercurrent immunoelectrophoresis after the patient with SLE in whom Smith antibodies, possibly as a result (CIE) or double diffusion (Table 2).16 it was first described in 1966.13 The of epitope spreading.14 If these antibodies are present, extractable nuclear antigen it targeted Anti-Smith antibodies have a low SLE generally is diagnosed within a was identified by immunodiffusion, sensitivity but high specificity for SLE. year.17 In patients with clinical fea- 12 • FEDERAL PRACTITIONER • APRIL 2011 CURRENT PERSPECTIVES ON RHEUMATIC LABORATORY TESTS tures of SLE, whose IIF-ANA tests are Table 1. Conditions associated with positive antinuclear positive, the anti-Smith antibody test is useful in confirming an SLE diag- antibodies nosis, though a negative anti-Smith Systemic Organ-specific test does not exclude SLE, and the autoimmune autoimmune Other Systemic anti-Smith test is not recommended disorders disorders Causes for distinguishing SLE from other au- toimmune disorders. Several studies • SLE • Hashimoto • Chronic infections: have shown that there is little corre- • Scleroderma thyroiditis hepatitis C, HIV, lation between anti-RNP antibodies • Polymyositis/ • Graves disease tuberculosis, and SLE disease activity,18,19 and the dermatomyositis • Autoimmune lepromatous anti-RNP test has proven unsuccess- • MCTD hepatitis leprosy, infectious ful in predicting organ damage in • RA • Primary biliary mononucleosis, 20 infective SLE. • Pauciarticular cirrhosis The anti-RNP test is, however, a juvenile • Primary autoimmune endocarditis diagnostic criterion for mixed con- chronic arthritis cholangitis • Malignancies: nective-tissue diseases (MCTD) be- • SS chronic lymphocytic leukemia, lymphoma cause affected patients have higher • Drug-induced lupus • Drugs levels of anti-RNP than patients with • Discoid lupus SLE. For MCTD, anti-RNP’s sensitiv- • Pregnancy ity ranges from 71% to 100%, and its MCTD = mixed connective-tissue disease; RA = rheumatoid arthritis; SLE = systemic lupus specificity ranges from 84% to 100%. erythematosus; SS = Sjögren’s syndrome. The Alarcon-Segovia criterion for MCTD specifies that the hemagglu- SJÖGREN’S SYNDROME (SS) tigens with properties similar to Ro tination titer of anti-RNP must be at ANTIBODIES and La, respectively.25 It was not until least 1:1600 to be diagnostic.21 In the late 1950s, Jones discovered, 1979 that Tan and colleagues discov- In addition to MCTD, anti-RNP in extracts from patients’ lacrimal and ered that Ro and La were identical to can be positive in patients with Rayn- salivary glands, a pair of antibodies SSA and SSB, respectively.26 aud phenomenon, systemic sclero- (later known as anti-SjD and anti-SjT) Two closely related proteins con- sis (SSc), and SLE. Anti-RNP and that precipitated the development of stitute the SSA/Ro antigen: 60 kDa— anti-Smith also can be found in the SS.23 In 1969, Clark and colleagues which is associated with small human absence of systemic autoimmune dis- described Ro (SjD) and La (SjT) as cytoplasmic RNAs, called hY RNAs— order, though it is relatively uncom- cytoplasmic antigens.24 In 1975, SSA and 52 kDa. The SSB/La antigen is mon.22 and SSB were identified as nuclear an- a 48 kDa protein that binds to vari- Table 2.
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