Which test would best differentiate between SLE and mixed connective tissue disease (MCTD)?

The ability to differentiate between systemic lupus erythematosus (SLE) and mixed connective tissue disease (MCTD) is greatly enhanced by the presence of specific autoantibodies associated with each condition. A multiplex or ELISA test for anti-SM and anti-RNP is particularly effective because these antibodies are distinct markers for SLE and MCTD, respectively.

Anti-SM antibodies are typically associated with SLE and are not usually present in MCTD, making their detection strong evidence for SLE. On the other hand, anti-RNP antibodies are commonly found in MCTD, but they can also be present in other conditions, including SLE. The simultaneous measurement of these antibodies allows for a clearer distinction between the two conditions, aiding in an accurate diagnosis.

Other testing methods, such as immunofluorescence testing using Crithidia as a substrate, primarily identify anti-double-stranded DNA antibodies, which are more specific for SLE but do not help in differentiating from MCTD. Slide agglutination testing lacks the specificity needed for these particular autoimmune diseases, as it is typically used for detecting blood group antigens or non-specific antibodies. Lastly, while laboratory tests can identify features common to these