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A substrate for the enzyme is applied, and catalysis by the enzyme leads to a change in color or fluorescence. ELISA results are reported as a number; the most controversial aspect of this test is determining the "cut-off" point between a positive and a negative result. A cut-off point may be determined by comparing it with a known standard.
Sometimes, retesting the donor in several months will produce a negative ELISA antibody test. This is why a confirmatory western blot is always used before reporting a "positive" HIV test result. [citation needed] Rare false positive results due to factors unrelated to HIV exposure are found more often with the ELISA test than with the western ...
Specimens with a reactive ELISA result are retested in duplicate. [116] If the result of either duplicate test is reactive, the specimen is reported as repeatedly reactive and undergoes confirmatory testing with a more specific supplemental test (e.g., a polymerase chain reaction (PCR), western blot or, less commonly, an immunofluorescence ...
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In the past nucleic acid tests have mainly been used as a secondary test to confirm positive serological results. [3] However, as they become cheaper and more automated, they are increasingly becoming the primary tool for diagnostics and can also be use for monitoring of treatment of viral infected individuals t.
Since the antibodies do not bridge between antigens, no agglutination occurs. Because no agglutination occurs, the test is interpreted as negative. In this case, the result is a false negative. The range of relatively high antibody concentrations within which no reaction occurs is called the prozone. [5]
Quantitative PCR (Q-PCR) is used to measure the quantity of a PCR product (preferably real-time, QRT-PCR). [2] It is the method of choice to quantitatively measure amounts of transgene DNA in a food or feed sample. Q-PCR is commonly used to determine whether a DNA sequence is present in a sample and the number of its copies in the sample.
ELISA is probably the second-most common serologic method. [21] The sensitivity of the ELISA was 100% when compared with blood culture, but only 44% compared with serologic tests other than ELISA. The specificity was >99%. In a study including 75 patients with brucellosis, five patients with positive ELISA had a negative tube agglutination test.