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The Kastle–Meyer test is a presumptive blood test, first described in 1903, in which the chemical indicator phenolphthalein is used to detect the possible presence of hemoglobin. It relies on the peroxidase -like activity of hemoglobin in blood to catalyze the oxidation of phenolphthalin (the colorless reduced form of phenolphthalein) into ...
A positive result induces a color change to pink. [4] Similar to the Kastle-Meyer test, a hemastix is also a catalytic test simplified to a specialized strip where the blood sample is extracted by a wet swab and placed directly on the hemastix. [7] A positive result induces a colour change from yellow to dark green. [7]
Most people who take a drug test take a presumptive test, cheaper and faster than other methods of testing. However, it is less accurate and can render false results. The FDA recommends for confirmatory testing to be conducted and the placing of a warning label on the presumptive drug test: "This assay provides only a preliminary result.
A positive test indicates the sample contains hemoglobin and, therefore, is likely blood. A false positive can result from the presence of substances with catalytic activity similar to hemoglobin. This test is not destructive to the sample; it can be kept and used in further tests.
The Kastle Meyer (KM) reagent is actually already basic in pH -- it is when the reduced phenolphthalein in the reagent is oxidized to phenolphthalein by the presence of heme (or some other oxidizer) that it is the phenolphthalein you are familiar with, which indeed, will turn pink in the presence of its alkaline solvent.
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The test was used in forensics for many years to test for the traces of nitroglycerine. Caustic soda is used to break down sample containing nitroglycerine to produce nitrite ions. The test involves the taking of a sample with ether and its division into two bowls. Caustic soda is added to the first bowl followed by the Griess reagent; if the ...
In fact, post-test probability, as estimated from the likelihood ratio and pre-test probability, is generally more accurate than if estimated from the positive predictive value of the test, if the tested individual has a different pre-test probability than what is the prevalence of that condition in the population.