Why are different biopsy procedures needed?

Biopsy is the resection (cutting out) of a piece of tissue for analysis. While many different tests for many conditions exist, sometimes they are limited in their ability to provide direct proof needed for a diagnosis. A biopsy will often demonstrate without a doubt the presence or absence of certain disease, and is often necessary as confirmation when the diagnosis in question would require serious treatment, such as surgery or chemotherapy. It is therefore most commonly employed to inspect tissues that are suspected of being cancerous. However, since a biopsy is a surgical procedure itself, it is usually not performed until evidence from less invasive tests suggests that the patient is likely to have the disease and biopsy is necessary. Admittedly, this has changed somewhat with the introduction of (expensive) genetic testing; however genetic testing suffers from our lack of understanding of the human genome, and thus can often only be used to demonstrate susceptibility to a disease or the presence of genes that contribute to the risk of developing it.