We present a case of a false-positive anti-
myeloperoxidase (MPO) antibody result on an
ELISA in a
patient with anti-
thyroid microsomal antibody (TMA)-positive
hypothyroidism. A 41-year-old
woman presented with
dyspnea on exertion. The initial evaluation revealed
pericardial effusion associated with
hypothyroidism. In addition, microscopic
hematuria with normal renal function and positive cytoplasmic
anti-neutrophil cytoplasmic antibodies (
c-ANCA) on immunofluorescent assay were found. In further evaluation, elevated anti-TMA and MPO
antibodies by
ELISA. While no definite signs of
vasculitis were present, the clinical
state improved with
thyroid hormone replacement and
diuretics. Anti-MPO antibody was still positive in the follow-up tests, and microscopic
hematuria persisted. On the basis of previous
reports that
thyroid peroxidase and MPO molecules contain cross-reactive
epitopes that are exposed in denaturated molecules, we suggest that in a
patient with anti-TMA-positive
hypothyroidism, anti-MPO antibody might also be positive on
ELISA without clinical signs of
vasculitis.