The polymerase chain reaction (PCR) is a powerful tool that is used in a wide variety of diagnostic procedures. PCR is used detect the presence of genetic material (DNA) that is unique and specific to an organism of interest. PCR works by amplifying a portion of DNA that is specific for that organism. This product can be easily visualized using standard laboratory procedures. The PCR test is very sensitive and can detect the presence of an organism when present at very low levels. For the diagnosis of bovine TB, PCR is used to identify Mycobacterium bovis in tissues collected at necropsy from animals suspected of being infected with bovine TB infection. PCR is only used on tissues that have histological (microscopic) evidence compatible with bovine TB. The results can typically be obtained within seven days and are classified as either positive or negative. Regardless of the results of the PCR test, tissues are submitted for the attempted culture of Mycobacterium bovis. However, a positive test obtained on PCR is highly suggestive that the animal is infected with bovine TB and, along with other test results, may be used to classify an animal as being infected with bovine TB.