Following you over from the Crohn's forum.
There is no definitive blood test for lupus. There are several that are correlated (to varying degrees) to having SLE, but there is no single test that determines it. Diagnosis is always based on a combination of several blood tests plus having multiple positive symptoms.
As for anti dsdna specificially, I think about
1/3 of people with SLE are positive for it. That means 2/3 of people with SLE are negative for it. On the other hand, only a small number of people have it positive but *don't* have SLE.
So if it's positive, SLE is highly suspected and needs to be considered. But if it's negative, SLE still can't be ruled out.
In your daughter's situation, it's best to have it measured along with measuring anti-histone antibodies.
Those results, in combination with her symptoms, should help determine whether it's SLE, drug-induced lupus, or neither.