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The mucus which is ejected from the lungs while coughing is called sputum. Sputum has several characteristics, such as color, consistency, and amount, which can be important for differential diagnosis of respiratory tract disorders. Sputum color in particular is one of the most commonly described parameters that can point to the type of bacterial, fungal, or viral infection, lung cancer, and other disorders.

However, the color of the sputum can provide only a rough prediction of a possible condition, and in order to reach the right diagnosis, other more specific tests are required. Here are some disorders which could be suspected based on the color of the sputum.

Colorless Sputum

Transparent, mucosal sputum usually means a viral infection. Viruses cause production of mucus, but they do not activate a large number of leukocytes to migrate to the site of the infection, so the sputum is usually colorless. This can also occur in some fungal infections.

Yellow Sputum

Most commonly, the sputum turns out to be yellowish. In otherwise healthy individuals, this usually means a bacterial infection. The yellow color stems from the high concentration of leukocytes in the place of infection, which “died” fighting against the bacteria that caused the infection. In cases of yellow sputum, it should be sent for microbiological assessment with antibiogram in order to determine the type of bacteria and the antibiotic to which it is sensitive.

In asthmatic persons, yellow sputum can be a result of accumulated eosinophilic cells (a subtype of leukocytes), which are mediators in inflammation of the airways associated with asthma. Some fungal infections of the respiratory tract can also cause yellow sputum.

Green Sputum

Some bacterial species can cause green sputum, such as Pseudomonas aeruginosa, Haemophilus influenzae, and Streptococcus pneumoniae. These bacteria contain specific enzymes that act on human tissues and produce a green color of the damaged tissue. In patients infected with Pseudomonas, the sputum can sometimes have different nuances – from green to dark blue.  These infections usually produce symptoms such as cough, a high body temperature, fatigue, sore throat, and chest pain. The treatment includes the appropriate antibiotics, according to the antibiogram, symptomatic treatment (analgesics, antipyretics, etc.), and rest.

Red Sputum

Red or reddish sputum usually means that there is free blood present in the lungs and airways. People commonly instantly relate this to lung cancer, but it is not necessarily true. Bronchitis of any kind can cause an irritation and mild bleeding of the inner layer of the bronchi, and if the blood appears in the sputum, the color can have different nuances from pink to bright red. However, lung cancer, tuberculosis, and mechanical injuries to the airways can also cause the appearance of blood in the sputum, so detailed examination is necessary in these cases.

Given that cough is officially the most common symptom in human medicine, sputum color cannot be taken as a parameter for distinction of any conditions without further examination. Moreover, obtaining sputum for analysis is not an easy task. Doctors need to instruct their patients on how to provide the sputum for examination, as it can only be obtained through the act of cough. It often happens that the material for examination turns out to be only saliva, which is not useful for microbiological examination, because the saliva doesn’t come from the respiratory system, but from the glands located in the mouth.

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