The Gram stain is a general stain used extensively in microbiology for the preliminary differentiation of microbiological organisms. The Gram stain is one of the simplest, least expensive, and most useful of the rapid methods used to identify and classify bacteria. The Gram stain is used to provide preliminary information concerning the type of organisms present directly from clinical specimens or from growth on culture plates. This stain is used to identify the presence of microorganisms in normally sterile body fluids (cerebrospinal fluid, synovial fluid, pleural fluid, peritoneal fluid). It is also used to screen sputum specimens to establish acceptability for bacterial culture (<25 squamous epithelial cells per field is considered an acceptable specimen for culture) and may reveal the causative organism in bacterial pneumonia.

Positive Gram stain results usually include a description of what was seen on the slide. This typically includes: Whether the bacteria are Gram-positive (purple) or Gram-negative (red) Shape — round (cocci) or rods (bacilli) Size, relative quantity (CFU/mL), and/or arrangement of the bacteria Whether there are bacteria present within other cells (intracellular) Presence white blood cells Fungi (in the form of yeasts or molds) may be seen on a Gram stain and are reported.