Autoantibody profile in diagnosis

Antinuclear antibodies are a hallmark of systemic autoimmune rheumatic diseases and can be found in a variety of clinical settings. However, their occurrence does not necessarily indicate the presence of any specific disease – they are thus sensitive but not specific diagnostically. Serology is of particular value in situations where clinical expression of the autoimmune rheumatic disease is incomplete, where the presence of a particular antinuclear antibody profile may be diagnostic. It is therefore imperative that requests for antinuclear antibody tests and the interpretation of the results be done in the light of clinical findings.

The indirect immunofluorescence test, using the HEp2 cell substrate, is the gold standard for detecting antinuclear antibodies (Table 21.1). In systemic lupus erythematosus and scleroderma, antinuclear antibodies can be detected in 95% or more of untreated patients with active disease by this method. In patients suspected of having an autoimmune rheumatic disease, the indirect immunoflourescence test is enough as a screening test for antinuclear antibodies. It is not cost‐effective to test automatically for anti‐dsDNA or other antibody specificities. The individual antinuclear antibody fluorescent patterns are of limited diagnostic utility but may provide guidance to more specific immunological tests. A false negative antinuclear antibody test result sometimes occurs if either the antigen is outside the nucleus (for example, anti Jo‐1 and anti‐ribosomal P protein antibodies, both often categorized under the umbrella term ‘antinuclear antibodies’) or if it is present in a form not recognized by a particular autoantibody (for example, when anti‐Ro is directed exclusively to determinants on the native Ro molecule not expressed in cultured HEp2 cells). In such cases, the clinical picture dictates that specific autoantibody assays should be undertaken.

Having detected antinuclear antibodies with the screening test, it is important to determine their specificity. This is part of the standard operating procedure of serology laboratories, but the process is greatly facilitated by the clinician giving sufficient clinical information when requesting the antinuclear antibody test. Specific antinuclear antibody tests are often helpful in stratifying patients into clinical subsets, which may be useful in the further management of specific clinical manifestations and prognostication (Table 21.2). These autoantibodies are usually present from the beginning of the clinical presentation and are detectable throughout the course of the disease. Some studies have shown that autoantibodies may be present for many years prior to clinical presentation. In some instances, such as the anti‐dsDNA test, autoantibody titres may fluctuate with disease activity. Many serology laboratories use commercial kits to detect specific autoantibodies and there is a move to using bead‐based multiplex immunoassays. Although the newer tests are less labour intensive, they vary in sensitivity and sometimes produce false‐positive results. This is especially the case with the anti‐dsDNA, anti‐Ro‐52 and anti‐Sm assays.

Which systemic autoimmune rheumatic disease?

Although the clinical presentation in the early stages can be similar between the systemic autoimmune rheumatic diseases, the evolution of typical clinical features over weeks or months usually distinguishes the characteristic patterns associated with the different diseases. Early diagnosis is aided by recognition of distinctive serological profiles that are generally present with the earliest clinical manifestations. Diagnosis can also be facilitated by typical laboratory abnormalities and histological changes in the tissues involved. For example, microscopic polyangiitis presenting with weight loss, fever, polyarthritis and active urinary sediment can be distinguished from lupus by an autoimmune response characterized by p‐ANCA antibodies directed against myeloperoxidase and the typical histological picture of pauci‐immune focal necrotizing glomerulonephritis. Similarly, clinically amyopathic dermatomyositis that presents with photosensitive eruptions on the face, arms and hands and is associated with myalgia can be distinguished from lupus by the distribution of the skin eruption, a raised serum creatine kinase, anti‐MDA5 and other myositis antibodies and typical changes on muscle biopsy, despite the absence of frank muscle weakness.

Diagnosis is often complicated if lupus is part of an overlap syndrome and the patient fulfils classification criteria for more than one systemic autoimmune rheumatic disease. Sometimes the overlap features are evident at initial presentation; at other times, the picture evolves sequentially. Patients who fulfil criteria for both systemic lupus erythematosus and rheumatoid arthritis are sometimes referred to as having ‘rhupus’. These patients are usually diagnosed as having rheumatoid arthritis initially because of typical rheumatoid features such as erosive arthritis, subcutaneous nodules and rheumatoid factor. An overlap syndrome should be considered when these features are accompanied by cutaneous, renal, haematological or other clinical manifestations characteristic of systemic lupus erythematosus and the presence of anti‐dsDNA antibodies.

Development of Sjögren’s syndrome during the course of systemic lupus erythematosus is well established but occasionally, patients with primary Sjögren’s syndrome develop typical features of lupus, especially photosensitive eruptions typical of subacute cutaneous lupus erythematosus, after many years of disease.

A distinctive type of overlap syndrome is that of ‘mixed connective tissue disease’ (Box 21.2). Here patients have an overlap of puffy fingers of early scleroderma (Figure 21.2), systemic lupus erythematosus, polymyositis, and a characteristic serological profile that includes high levels of antibodies to U1RNP. However, there is much controversy as to whether this is a distinct systemic rheumatic disease, with critics and protagonists.