Confirming widespread beliefs by doctors and parents alike, the color of phlegm coughed up by people is indeed a good indicator of whether that person has a bacterial infection, an international group of researchers found.
Green or yellow "sputum," as clinicians call it, more often than not reflects a bacterial infection, whereas clear, white or rust colored 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 Center in Seattle, who was not involved in this study.
That's because the researchers found that the great majority of the time, clear phlegm did not harbor disease-causing bacteria.
Antibiotics treat bacterial infections, but don't work on virus-caused illnesses like the common cold.
The team analyzed 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.
Green sputum had bacteria in it 59 out of every 100 times, and yellow sputum had bacteria in it 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 color 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 colored snot to be a marker of a bacterial infection.
The olive or pea-like hue comes from a green-colored enzyme called myeloperoxidase, which is involved in fighting infections.
Some researchers have called into question the practice of prescribing antibiotics based on phlegm color.
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.
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 colored 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.