Reason for culture coming back negative on cellulitis?

Answer 1

Cultures look for bacterial growth. And thus in order for bacteria to grow in a given culture medium LIVE bacteria are needed. Depending on the mechanism for obtaining a sample, and the sample placed in the culture medium (i.e. swab, probe, biopsy, blood, etc.) the probability of obtaining live bacteria varies.

Pus that usually accompanies an infection is composed of dead bacteria within white blood cells. And thus the swab of a skin wound full of pus is UNlikely to yield a positive (defined by bacterial growth in the culture medium) result.

In short live bacteria is needed for a culture to be reported as positive. Not all samples taken for culture contain live bacteria, even in the presence of an infection (it is just the live bacteria are too deep to be accessed by the sampling technique used.

I hope this answers your question.

Sincerely,

Matthew C. Lee, MD, RPh, MS

Answer 2

A culture may come back negative for cellulitis if the infection is caused by a non-bacterial (e.g., fungal) pathogen, if the sample collected was contaminated, or if the infection is due to a non-infectious cause (e.g., allergic reaction). Additionally, if antibiotics were started before the culture was obtained, this could also result in a negative culture.