Two hundred eighty-eight serum specimens from 144 asymptomatic menopausal women were assayed two times each for CA 125. The specimens were randomly assigned to sixteen CA 125 kits. Our purpose was to quantitate the biological variability and the assay variability that occurs at low levels of this tumor marker. All of the subjects had CA 125 values less than 35 units/ml. The total error from all sources was about 13% of the observed values. These findings suggest that single, low CA 125 values are reliable indicators of a woman's true CA 125 value. Clinical trials of ovarian cancer screening that incorporate CA 125 measurements should evaluate the performance of this test at thresholds lower than the usual 35 units/ml.
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