The antinuclear antibody (ANA) test is positive in almost all people who have systemic lupus erythematosus (SLE). It is also positive in some patients who do not have SLE
The anti-deoxyribonucleic acid (DNA) test is positive only in people with SLE but not in all who do. Which of the following is true?
a. The ANA test is a good screening test, and anti-DNA test is a good confirmatory test.
b. The anti-DNA test is a good screening test, and the ANA test is a good confirmatory test.
c. Both are good screening tests for SLE.
d. Neither of these tests is valid.
ANS: A
Because almost all patients with SLE have a positive ANA test, it is a good screening tests (if the result is negative, then it practically rules out this diagnosis for a patient). However, because it is also positive in other patients, the anti-DNA test is a good confirmatory test, because only patients with SLE have a positive result. In practice, the ANA test is done first; if it is positive, then the anti-DNA test is done as follow-up. If a patient is positive with both tests, then his or her diagnosis is SLE.
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