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Green phlegm linked to bacterial infection

15th November 2011

The colour of people's sputum indicates whether an infection is bacterial, according to a recent Spanish study.


While doctors and parents have long held the belief that phlegm colour was an indicator of a bacterial infection, the recent study adds to the scientific basis for that belief.

Green or yellow phlegm most often reflects a bacterial infection, whereas phlegm that is clear only does so in a minority of cases.

The finding may give doctors a reason not to prescribe antibiotics, without needing to test for a bacterial infection.

Neil Hampson, a professor emeritus at Virginia Mason Medical Centre in Seattle, in the US, who was not involved in the study, said that people who complained of clear or white sputum production had no reason to spend money on antibiotics.

Hampson conducted a study several years ago which established a similar result to the recent study, although he did not study chronic bronchial conditions.

For the study, the researchers analysed sputum from more than 4,000 people.

All of the study subjects had chronic bronchitis at the time, so they were producing a lot of phlegm.

The researchers found that, about 59% of the time, green sputum had bacteria in it. Yellow phlegm harboured bacterial specimens about 46% of the time.

But clear or white phlegm was nearly always free from pathogenic bacteria. Only 18% of clear or white phlegm harboured bacterial specimens.

Lead researcher Marc Miravitlles, a researcher at the Hospital Clinic in Barcelona, Spain, said that sputum culture was not readily available in most clinical practices.

He said that the simple inspection of the sputum may be a cheap and quick way to check people for bacterial infections.

The reason sputum can appear green in the first place has to do with an enzyme known as myeloperoxidase, involved in immune activity.

Myeloperoxidase helps cells protect themselves by producing tiny amounts of destructive material that kills bacteria, and makes use of an iron-containing pigment.

Miravitlles said that the study's results did not apply to occasional respiratory infections, but rather to chronic bronchial infections.

Hampson said that, in his experience, parents and individuals should not get overly worried about phlegm that was clear or white.

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