Reasons for a False-Positive Gram Stain

A Gram stain may give false-positive results if certain problems occur during the preparation of the smear.

Situation: The Gram-stained slide shows “bacteria” that are not actually in the specimen.

A negative control can be helpful in detecting the proble,.

Situation	Cause	Prevention
Bacteria Present	dirty slides	Precleaned slides.
	contaminated stain	Filter.stain. Avoid contamination. Change.
	contaminated water	Clean water..
	cross-contamination	Prepare and stain carefully.
	mislabeled slide or specimen	Be careful labeling specimens and slides.
Pseudo-bacteria Present	stain precipitate	Often seen in areas away from the specimen itself.
	tissue or debris misinterpreted

where:

• A box of glass slides from China showed enteric bacteria covering the surface. One can never assume that glass slides are clean.

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