Final answer:
The Indirect Immunofluorescence Assay (IFA) for antinuclear antibodies (ANAs) has replaced the LE cell preparation in diagnosing Systemic Lupus Erythematosus (SLE).
Step-by-step explanation:
The antibody test that has replaced the LE cell preparation in the diagnosis of Systemic Lupus Erythematosus (SLE) is the Indirect Immunofluorescence Assay (IFA) for the detection of antinuclear antibodies (ANAs). The IFA test involves fixing cultured cells to a slide, permeabilizing them to allow for antibody penetration, incubating with the patient's serum followed by a fluorescently-labeled anti-human IgG antibody. This test is vital because ANAs are present in over 95% of SLE patients.
A positive IFA for ANA, combined with symptoms affecting two or more organ systems and a titer of at least 1:40, according to the American College of Rheumatology, suggests SLE. Autoantibodies such as anti-double-stranded DNA (ds-DNA) and anti-Sm are also highly specific to SLE, and their presence strongly supports the diagnosis, corroborating the results of the ANA IFA.