Indications for Testing
Laboratory testing for MCTD is appropriate to:
- Diagnose individuals who present with rheumatologic disease with overlap features
- Monitor patients with an established diagnosis of MCTD for the presence of new antibodies that might indicate the evolution of MCTD to another connective tissue disease
- Monitor patients with MCTD for disease progression, particularly lung disease
Criteria for Diagnosis
Various diagnostic criteria have been proposed for MCTD; however, only the Alarcón-Segovia criteria and the Kasukawa criteria have been regularly used. To fulfill the Alarcón-Segovia criteria, patients must have significantly elevated anti-U1 RNP antibody levels, in addition to three or more of the following signs and symptoms: edema of hands, synovitis, myositis, Raynaud phenomenon, and acrosclerosis. The Kasukawa criteria have been used mostly in pediatric populations, and take into account additional findings such as lung and esophageal involvement.
The presence of ANAs is a classic feature of SARDs ; therefore, ANA testing is a useful initial test for suspected MCTD. However, ANAs are not specific to SARDs and are also seen in infections and malignancies, and even in healthy individuals. For that reason, ANA testing is best used for patients with a strong likelihood of MCTD or another SARD.
ANA testing using the IFA test is considered the gold standard method for detecting these antibodies. ANA IFA test results are generally reported with patterns and associated titers. Positive results for ANAs using solid-phase immunoassays (eg, enzyme-linked immunosorbent assays [ELISAs], multiplex bead assays) have limited utility for diagnosis because ANA patterns that may have relevant antigenic and disease associations can only be observed via IFA assays. Generally, the ANA patterns may help guide confirmatory testing for specific autoantibodies or may suggest the presence of a certain SARD in the absence of confirmatory tests. For comprehensive information on patterns and their clinical associations, refer to the International Consensus on ANA Patterns website.
Extractable Nuclear Antigen Antibodies
Extractable nuclear antigens (ENAs) include more than 100 different soluble cytoplasmic and nuclear antigens. The most commonly used ENA tests include tests for Smith, Smith/RNP (Sm/RNP, or U1 RNP), SSA-52 (Ro52), SSA-60 (Ro60), SSB, Scl-70 (topoisomerase 1), and Jo-1 antibodies. These are usually the first-line tests following a positive ANA result. Thus, panels that test for specific ENAs can be helpful in the evaluation of SARDs.
Sm/RNP (U1 RNP) is an ENA associated with MCTD. Anti-U1 RNP antibodies are a hallmark of MCTD and are found in the majority of patients with the disease. Their presence can help to distinguish MCTD from other autoimmune diseases with similar clinical features, such as SLE, polymyositis, and SSc. All major clinical criteria for MCTD require the presence of anti-U1 RNP antibodies. In addition, anti-U1 RNP antibody levels have been reported to correlate with disease activity.
Additional Specific Antibodies Associated with MCTD
Based on the clinical features of MCTD, patients may present with other SARD-associated antibodies. These may include anti-double-stranded DNA (anti-dsDNA), anti-Smith, anti-SSA (anti-Ro), ribosomal-P, anticyclic citrullinated peptide (anti-CCP), rheumatoid factor (RF), anti-beta-2 glycoprotein 1 (anti-β2GPI), anticardiolipin (aCL), and antiendothelial cell antibodies.
Additional laboratory tests might include a CBC to evaluate patients for mild anemia, leukopenia, and thrombocytopenia, which are common features in MCTD. They also might include a C-reactive protein (CRP) test to detect inflammatory processes. If CRP testing is unavailable, erythrocyte sedimentation rate (ESR) testing can be used, but CRP is considered to be a more sensitive and specific test for acute phase inflammation.
Patients with MCTD should be monitored for the development of lung disease, one of the most serious complications of MCTD. Lung disease in MCTD can take different forms, eg, interstitial lung disease (ILD) or pulmonary hypertension. Early detection may slow or arrest the development of lung fibrosis and related complications. Pulmonary function tests, such as diffusion capacity for carbon monoxide (DLCO) tests, can be used to assess lung function.
There is some evidence that serum levels of N-terminal pro-brain natriuretic peptide (NTproBNP) will be increased in patients with MCTD who develop pulmonary hypertension; therefore, NTproBNP testing may be helpful in patients with MCTD who have symptoms that suggest pulmonary hypertension.
The clinical characteristics of MCTD in a given patient can change over time so that MCTD is eventually classified as a different SARD. Termed “clinical shift,” this disease evolution has been reported in up to 50% of patients with MCTD during the decade after diagnosis. Some investigators have reported an association between particular antibodies and clinical shift from MCTD to other SARDs. Long-term monitoring of patients with MCTD is recommended to assess disease course and possible progression to another connective tissue disease.