What Does Gram Stain Mean on a Lab Report?

What it means

In a Gram stain, a lab scientist spreads a sample onto a glass slide and adds a series of dyes. Bacteria with a thick outer wall hold on to a purple dye and are called Gram-positive. Bacteria with a thinner wall lose the purple and take up a pink dye, so they are called Gram-negative. The scientist also notes their shape, such as round (cocci) or rod-shaped (bacilli), and whether white blood cells are present.

A Gram stain gives an early clue within hours, long before the germ can be grown and named. It helps the medical team choose a sensible antibiotic while they wait for the full culture result. On a report, it is usually the first line of a microbiology result, followed later by the name of the germ and which antibiotics work against it.

A Gram stain is not the same as a culture. The stain shows what the bacteria look like, while a culture grows them so the lab can name them and test antibiotics on them.

Where you may see it

• The first update on a positive blood culture result
• Microbiology reports for urine, sputum, wound swabs or joint fluid
• Spinal fluid results when meningitis is being checked
• Hospital notes such as "Gram stain: Gram-positive cocci in clusters"