The antibody test does not directly detect the presence of the AIDS virus. Rather, by identifying antibodies typically associated with the body's countering of the virus, the tester infers the subject's exposure to the virus. The current HTLV-III test for screening donated blood and organs is the enzyme-linked immunosorbent assay (ELISHA). The ELISHA test is not 100-percent accurate, but the more expensive Western blot test is believed to be 100-percent accurate. A strongly positive ELISHA test should be repeated and then followed by a Western blot test for confirmation. A false positive test result is possible with any laboratory test, however. This may be due to the accidental positivity produced by structurally similar but unrelated antibodies, differences in test kits, a weakly positive test, and statistical variation inherent in any test of this nature. False negatives may result from error in the test's administration, variation in test kits, and the time frame following exposure, since antibody levels fall late in the course of AIDS. The use of the HTLV-III test for diagnosis or prognosis is inadequate.