Updated 13 January 2017

The colour of phlegm can show if you have an infection

Confirming widespread beliefs by doctors and parents alike, the colour of phlegm coughed up by people is indeed a good indicator of whether that person has a bacterial infection.

Green or yellow sputum, as clinicians call it, more often than not reflects a bacterial infection, whereas clear, white or rust coloured phlegm most likely does not, according to the new study.

The results could help doctors determine whether or not a patient would benefit from antibiotics.

When someone comes in and complains of sputum production and it's clear or white, there's no reason to spend money on antibiotics, said Dr Neil Hampson, a professor emeritus at Virginia Mason Medical Centre in Seattle, who was not involved in this study.

Antibiotics don’t treat colds

That's because the researchers found that the great majority of the time, clear phlegm did not harbour disease-causing bacteria.

Antibiotics treat bacterial infections, but don't work on virus-caused illnesses like the common cold.

The team analysed sputum from more than 4,000 people with chronic bronchitis.

Just 18 out of every 100 samples of clear phlegm tested positive for disease-causing bacteria.

The study, published in the European Respiratory Journal, did not show that green or yellow phlegm warrants an antibiotic prescription every time, however.

Cheap and quick

Green sputum had bacteria in it 59 out of every 100 times, and yellow sputum had bacteria 46 out of every 100 times.

Dr Marc Miravitlles, the lead author of the study and a researcher at the Hospital Clinic in Barcelona, Spain, said sputum colour is not as good an indicator of bacterial infections as testing the sample in a laboratory.

But sputum culture is not readily available in most clinical practices, Miravitlles wrote in an email to Reuters Health. Therefore, the simple inspection of the sputum may be a surrogate marker of infection: cheap and quick.

Many parents and physicians have long considered coloured snot to be a marker of a bacterial infection.

Antibiotic prescription questioned

The olive or pea-like hue comes from a green-coloured enzyme called myeloperoxidase, which is involved in fighting infections.

Some researchers have called into question the practice of prescribing antibiotics based on phlegm colour.

A recent study found that people with green or yellow phlegm who received antibiotics didn't recover any faster from their sickness than people without mucous and who didn't get the medications (see Reuters Health story of March 29, 2011).

White sputum no problem

Miravitlles said his results do not apply to people with the occasional respiratory illness, because his study looked only at people with long term bronchitis.

A study by Hampson from three years ago, however, found similar results among patients who coughed up mucus, but didn't necessarily have chronic bronchitis.

While it's not clear whether people with coloured phlegm would benefit from antibiotics, my advice to the person at home or to the parent of a child is that if the sputum is clear or white, they shouldn't be as concerned, Hampson said.

(Reuters Health, November 2011) 

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Professor Keertan Dheda has received several prestigious awards including the 2014 Oppenheimer Award, and has published over 160 peer-reviewed papers and holds 3 patents related to new TB diagnostic or infection control technologies. He serves on the editorial board of the journals PLoS One, the International Journal of Tuberculosis and Lung Disease, American Journal of Respiratory and Critical Medicine, Lancet Respiratory Diseases and Nature Scientific Reports, amongst others. Read his full biography at the University of Cape Town Lung Institute.

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