What it is: Anti-dsDNA is a protein directed against double-stranded DNA. DNA is the material that makes up the body's genetic code.
Why the test is used: Between 75% and 90% of people with lupus have a positive anti-dsDNA test. Also, the test is very specific for lupus. Therefore, a positive test can be useful in confirming a diagnosis. For many people, the titer, or level, of the antibodies rises as the disease becomes more active. So, doctors can also use it to help measure disease activity. Also, the presence of anti-dsDNA indicates a greater risk of lupus nephritis, a kidney inflammation that occurs with lupus. So a positive test can alert doctors to the need to monitor the kidneys.
Limitations of the test: Up to 25% of people with lupus have a negative test. So, a negative test doesn't mean a person doesn't have lupus.
Anti-Ro(SSA) and Anti-La(SSB)
What it is: Anti-Ro(SSA) and Anti-La(SSB) are two antibodies that are commonly found together. They are specific against ribonucleic acid (RNA) proteins.
Why the test is used: Anti-Ro is found in anywhere from 24% to 60% of lupus patients. It's also found in 70% of people with another autoimmune disorder called Sjögren's syndrome. Anti-La is found in 35% of people with Sjögren's syndrome. For this reason, their presence may be useful in diagnosing one of these disorders. Both antibodies are associated with neonatal lupus, a rare but potentially serious problem in newborns. In pregnant women, a positive Anti-Ro(SSA) or Anti-La(SSB) warns doctors of the need to monitor the unborn baby.
Limitations of the test: Like other antibodies, the fact that the test is not positive in many people with lupus means it can't be used to diagnose lupus. Also, it is more indicative of Sjögren's syndrome than of lupus.
C-Reactive Protein (CRP)
What it is: CRP is a protein in the body that can be a marker of inflammation.
Why the test is used:The test looks for inflammation, which could indicate active lupus. In some cases, the test could be used to monitor inflammation. Results of the test could indicate changes in disease activity or in response to treatment.
Limitations of the test: Because there are many causes for an elevated result, including infection, the test is not diagnostic for lupus. Nor can it distinguish a lupus flare from an infection. Also, the level of CRP doesn't directly correlate with lupus disease activity. So it isn't necessarily useful for monitoring disease activity.