What is being tested?A Gram stain is used to determine if
bacteria are present in an area of the body that is normally sterile, such as spinal fluid. A sample from the infected area is smeared on a glass slide and allowed to dry. A series of stains are applied and then the stained slide is examined under a microscope where bacteria appear either purple (gram positive) or pink (gram negative). The test is named for Dr. Christian Gram, who invented the process.
A Gram stain can predict the type of bacteria causing an infection, such as pneumococcal pneumonia or a staphylococcal abscess. Viruses cannot be seen with a Gram stain since they lack the cell wall, which takes up the stain.
