Approach to rheumatology test

We can do tests for the probable dx anticipated. It is prudent to order specific tests if indicated. Some of these tests sre of general nature and are best screeners.

In the case of the RF test, it is not specificto JIA or to the childhood equivalent (RF-positive polyarticular JIA), which is a relatively infrequent arthritis in children.

The RF test is often nonspecifically positive with infectious illnesses, including common infections as well as less common infections that can be associated with persistent problems, such as subacute bacterial endocarditis.

It can also be positive in rheumatic diseases other RSlike SLE and MCTD.

Similarly, the ANA test is not specific and most commonly is not an indication of a health problem. In children, the background rate of positive results is approximately 25%, most often triggered by minor infectious illnesses.

It is a commonly positive result in many subtypesof JIA,but its utility is not for diagnosis but rather for identifying the risk of developing auveitis and establishing the frequencyof screening ophthalmologic examinations.

*As mentioned,itis virtually always positive in SLE, and although it is an important confirmatory test for SLE, a negative result is what is often most informative because it makes this diagnosis highly unlikely*.

When there is a positive result, and SLE or a similar condition associated with specific ANA is suspected, then testing for specific ANAs can help clarify a diagnosis.

The target antigens in the nucleus include native (double-stranded) DNA, or RNA-protein complexes. The physician can order a specific screening test for anti-dsDNA antibody and then screen for the other targets using an antiENAantibody panel.

Antibodies against dsDNA are specific for lupus, as is the Smith antibody specificity in the antiENA antibodypanel.Because these tests detect only a subset of specific ANAs, there is no reason to order these when the screening ANA result is negative.

Antibodies specific to scleroderma are commonly included in theanti-ENA antibody panel and are associated with systemic scleroderma.

Antibodies associated with Sjögren syndrome are also part of the ENA panel but are not entirely specific to Sjögren and can be seen in SLE and other more limited forms of lupus, including neonatal lupus.

Finally, anti-RNP antibody is a part of theENA panel and occurs at lower values inSLE,but whenit is present at high values and is the only positive specific antibody, anti-RNP suggests the diagnosis of MCTD, an interesting overlap condition that can have features of RA (including RF), SLE (but usually not renal disease), DM, and scleroderma.

Mixed connective tissue disease should be considered in the list of possible diagnoses whenever a patient has RP or symptoms and signs suggestive of SLE, DM, RA, or scleroderma. Suspicion of MCTD should prompt ordering of a screening ANA and, if positive, then specific ANAtesting.

About Dr. Jayaprakash

Asst. Prof. of Pediatrics, ICH. Institute of Child Health. Gov. Medical College Kottayam. Kerala, India.

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