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.
This information is useful in guiding initial antimicrobial therapy.
Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test
The Gram stain is a general stain used extensively in microbiology for the preliminary differentiation of microbiological organisms. The Gram stain is 1 of the simplest, least expensive, and most useful of the rapid methods used to identify and classify bacteria.
Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.
No organisms seen or descriptive report of observations.
Gram-negative organisms do not retain crystal violet or iodine which are subsequently washed out by acetone/alcohol. Subsequently, they appear red because they are only stained with safranin dye. On the other hand, gram-positive organisms retain the crystal violet iodine complex after decolorization with acetone/alcohol and, therefore, stain purple.
Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances
Over decolorization may result in the loss of the crystal violet iodine complex from gram-positive organisms and result in a misinterpretation.
Clinical Reference Provides recommendations for further in-depth reading of a clinical nature
Forbes BA, Sahm DF, Weissfeld AS: Bailey and Scott’s Diagnostic Microbiology. 11th edition. Mosby-Elseiver, St. Louis, MO 2002, pp 122-125