Gram Stain
Definition
A laboratory test that groups the bacteria in a sample taken from a suspected site of infection by their color and shape, helping identify which type of bacteria is causing the infection.
Purpose
Detecting bacterial infections and identifying what is causing the illness and the best treatment. It can also help identify certain fungal infections.
Results
Bacteria are classified by how they respond to staining:
| Group | Color | Example infections |
|---|---|---|
| Gram positive | Stays purple | MRSA, streptococcal infections |
| Gram negative | Turns pink/red | Pneumonia, urinary tract infection, bacterial meningitis |
The shape of the bacteria, round (cocci) or rod-shaped (bacilli), is also seen, which gives a further clue for diagnosis.
Risks and Limitations
Gram stain cannot identify the exact species of bacteria, a culture is needed for a definitive diagnosis and the right antibiotic choice. It cannot detect viruses. Also, if the number of bacteria is low, the result can come back negative even though an infection is present.