GRAM - Clinical: Gram Stain

Test Catalog

Test Name

Test ID: GRAM    
Gram Stain

Useful For Suggests clinical disorders or settings where the test may be helpful

Identifying microorganisms in normally sterile body fluids


Screening sputum specimens for acceptability for bacterial culture 


Guiding initial antimicrobial therapy

Testing Algorithm Delineates situations when tests are added to the initial order. This includes reflex and additional tests.

See Infective Endocarditis: Diagnostic Testing for Identification of Microbiological Etiology in Special Instructions.

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 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.

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.

Interpretation Provides information to assist in interpretation of the test results

During the staining process, the crystal violet and iodine form a complex within the heat fixed cell. In gram-negative organisms, this complex is readily washed out by the acetone-alcohol. They appear red because they retain only the safranin dye (counterstain). Gram-positive organisms retain the crystal violet-iodine complex after decolorization and remain purple.


Cells and Organisms will be reported according to the following tables: 


White Blood Cells

Epithelial Cells




Power Field


Rare (R)

< or =1

Oil Immersion Field


Rare (R)


Few (F)


Few (F)


Moderate (O)


Moderate (O)


Many (M)


Many (M)


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 Recommendations for in-depth reading of a clinical nature

Atlas RM, Snyder JW: Reagents, stains, and media: bacteriology. In Manual of Clinical Microbiology. Vol 1. 10th edition, Edited by J Versalovic. Washington DC, ASM Press, 2011, pp 272-307

Special Instructions and Forms Library of PDFs including pertinent information and consent forms, specimen collection and preparation information, test algorithms, and other information pertinent to test