An LE prep (lupus erythematosus preparation) is medical test where blood is taken out of the patient, agitated (usually with small glass beads) and left to stand. Damaged cells will release their nuclei, which get coated with anti-nuclear antibodies (if present, as in the case of lupus or other autoimmune dieases). A blue dye is put in so that the clumpted nuclei are stained blue. These clumps are subsequently phagocytosed by Fc-bearing intact monocytes and neutrophils, which produces blue-stained monocytes and neutrophils (the LE cells). The test is positive in 70-90% of SLE patients.
LE prep has largely been replaced by the more sensitive ANA titers.