Sputum: what it is and where it comes from in the lungs and bronchi

You coughed up a little mucus and wondered whether everything is fine. Mucus is always present in the airways, while sputum is what is coughed up during pathological processes. Its appearance, consistency, odor, and volume help the doctor notice what is happening in the airways.

What sputum is and how it differs from mucus

Sputum (Latin sputum) is a pathological secretion of the tracheobronchial tree expelled during expectoration, mixed with saliva and secretions of the mucous membrane of the nasal cavity and paranasal sinuses.

Normally, the glands of the trachea and large bronchi produce mucus with cellular elements, mainly alveolar macrophages and lymphocytes. It has a bactericidal effect, and the ciliated epithelium, through mucociliary clearance, removes inhaled particles and cellular debris. Normal secretion does not exceed 100 milliliters per day and is swallowed by a person as it is produced, whereas sputum is a pathological secretion expelled during expectoration.

Where sputum comes from in the lungs and bronchi

In pathological processes, the volume of sputum produced can range from a few milliliters to one and a half liters per day. Cough is most often caused by a cold or flu. Other causes include smoking, acid reflux with heartburn, allergies such as hay fever, infections such as bronchitis or COVID-19, and mucus dripping down the back of the nose into the throat. Cough is rarely a sign of something serious, such as lung cancer.

What the appearance of sputum shows

Expectoration helps clear the airways. The doctor uses the color, consistency, odor, and volume of sputum as signs of various conditions.

• Mucous — colorless and transparent: catarrhal inflammation, the onset of an acute process, or chronic inflammation in remission.
• Mucopurulent — viscous, yellowish or greenish: bronchitis, pneumonia, bronchiectasis, pulmonary tuberculosis.
• Purulent — semi-liquid, yellowish-greenish, foul-smelling: abscess, purulent bronchitis, lung gangrene.
• Serous — colorless, liquid, foamy: alveolar pulmonary edema, sometimes pink.

Rusty sputum occurs in lobar pneumonia, and black sputum in pneumoconioses. Streaks or clots of blood (hemoptysis) may be observed in various diseases. Sputum usually has no odor; a putrid odor occurs in gangrene or lung abscess.

Allergy, asthma, and characteristics of sputum

In allergy, for example hay fever, cough is among the causes. In bronchial asthma and eosinophilic infiltrative process, sputum can be bright yellow, canary-colored: the shade is due to a large number of eosinophils. Sputum can be liquid, thick and viscous, as well as mucous, serous, mucopurulent, and purulent; color, consistency, odor, and volume are of medical importance.

How sputum is tested and when to see a doctor

The doctor may listen to the chest with a stethoscope, take a sputum sample, order an X-ray, an allergy test, or a lung function test, and rarely a referral to a hospital.

Sputum culture is a test that looks for infectious agents. During the test, the patient is asked to cough deeply and spit sputum into a special container: the thicker the sample, the higher the chance that it reflects secretions from the lower airways. In the laboratory, part of the sample is placed in a dish and observed for two to three days or longer. Normally there are no pathogenic microbes; sometimes bacteria that usually live in the oral cavity grow in the culture. A positive result helps diagnose bronchitis, pneumonia, tuberculosis, an exacerbation of COPD, or cystic fibrosis.

An ordinary cough resolves on its own within 3–4 weeks. See a doctor if the cough lasts longer than 3 weeks, you are losing weight without reason, or your immunity is weakened, for example due to chemotherapy or diabetes.

Sputum by itself is not a reason to call an ambulance. Call 103 or 112 if you are coughing up blood, have difficulty breathing, chest pain, feel very unwell, the cough is rapidly worsening, or the side of your neck is swollen and painful.

This material is for reference: it helps notice details but does not replace a doctor's consultation.

This content is for reference and does not replace a doctor's consultation. Medical disclaimer