Is testing for antihistone antibodies clinically useful to distinguish systemic DILE from idiopathic SLE in a patient taking either procainamide or hydralazine?
Testing for antihistone antibodies can occasionally be useful in situations in which the diagnosis of DILE is being considered. As discussed, nearly all patients (95%) with symptomatic procainamide- or hydralazine-induced lupus demonstrate elevated serum levels of IgG antihistone antibodies. Thus, a negative test would make this diagnosis unlikely. However, a positive test for antihistone antibodies has much less diagnostic value because 50% to 80% of patients with active idiopathic SLE also have a positive test. Furthermore, some patients taking either procainamide or hydralazine will have a positive test but not have symptoms of a lupus-like disease. Asymptomatic patients tend to have IgM and not IgG antihistone antibodies. In most cases in which DILE is being considered, performing an ANA test and if positive (usually in a homogenous pattern), taking the patient off the offending agent may be the most cost-effective approach to the situation.