The Consumer Product Safety Commission said that they didn't find home test kits useful for detecting lead because in their test, they found too many false negatives and a few false positive test results.

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Although the contaminated bills do not affect health, Negrusz said, they could cause in a false positive drug test if a person, such as a law enforcement officer or banker, handles contaminated currency repeatedly.

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The real false positive rate for this test is not known yet.

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If one in 1, 000 people have a disease and the false-positive rate for a test is 5%, then the probability that a person testing positive has the disease is at most 2%.

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Given that history, the odds were pretty high back in 1989 that the test result he received was a false positive.

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It suggests the test is far more likely to give 'false-positive' readings from cervical, or Pap, tests, than pick up early signs of cancer.

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The test is pretty reliable at detecting the disease, returning a false positive in just 1 in 5, 000 tests (a 99.98 percent accuracy).

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Because some legal substances -- such as fertilizers and heart medicines -- can result in "false positives, " Stanley said the ACLU also wants to ensure that people who test positive be treated respectfully.

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According to research compiled by the U.S. Agency for Healthcare Research and Quality, the percentage of false-positive readings is between 7% and 8% for women aged 40 to 59 who took the test.

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