The Color of Your Mucus Could Offer Clues About Bronchiectasis Activity

For people living with bronchiectasis, the color of the mucus they cough up may say more about their lung health than once thought. A new analysis of a large clinical trial found that darker, more discolored sputum was tied to higher levels of airway inflammation, more flare-ups and more lung damage.

The findings come from a post hoc analysis of the phase 3 ASPEN trial (NCT04594369), presented at the CHEST 2026 annual meeting held Oct. 18 to Oct. 21 in Phoenix, Arizona.

Inflammation at the root of bronchiectasis

Bronchiectasis is a chronic lung condition in which the airways become permanently widened, thickened or scarred. This makes it harder to clear mucus, leaving the lungs prone to infection and repeated flare-ups.

A key driver of the disease is inflammation fueled by neutrophils, a type of white blood cell. Neutrophils release enzymes called neutrophil serine proteases, including neutrophil elastase, cathepsin G and proteinase 3. These enzymes help fight infection, but in excess they can damage the airways over time.

Sputum color has long been considered a possible indicator of inflammation and disease activity. The new analysis set out to test how closely the color patients report matches what’s happening at the cellular level.

How researchers tracked sputum color

ASPEN was a global, randomized, double-blind, placebo-controlled trial. It enrolled adults with non-cystic fibrosis bronchiectasis who had at least two flare-ups in the previous 12 months. Participants were randomly assigned to take brensocatib 10 mg, brensocatib 25 mg or placebo once daily for 52 weeks.

Each day, participants rated their sputum color on a six-point scale using the bronchiectasis exacerbation and symptom tool (BEST) diary. For this analysis, researchers grouped those ratings into three categories: mucoid (clear or white), mucopurulent and purulent (more discolored). They also measured inflammatory enzyme levels in sputum samples from a subset of participants through week 56.

The analysis included 326 patients from all three study arms who had both sputum color and enzyme data.

Darker sputum matched higher disease activity

People with more purulent sputum had higher levels of all three inflammatory enzymes. Darker sputum was also linked to several signs of more active disease, including:

• Higher numbers of neutrophils in sputum
• More frequent infection with Pseudomonas aeruginosa, a bacterium associated with more severe bronchiectasis
• Higher flare-up rates
• Higher scores on CT scans, reflecting more airway damage

Together, the results suggest that the sputum color patients notice day-to-day reflects real differences in airway inflammation.

“Sputum is a noninvasive marker of airway inflammation, with purulence reflecting the presence and activity of neutrophils in the airways,” said James Chalmers, MBChB, Ph.D., a presenting author of the study.

Brensocatib reduced inflammation across all color groups

Brensocatib, sold as Brinsupri, became the first U.S. Food and Drug Administration (FDA)-approved treatment for non-cystic fibrosis bronchiectasis in 2025. It works by blocking an enzyme called dipeptidyl peptidase 1 (DPP1), which neutrophils need to activate their serine proteases.

Compared with placebo, brensocatib lowered neutrophil elastase levels in all three sputum color groups. Sputum became more discolored during flare-ups in every study arm. However, during severe flare-ups, people taking either dose of brensocatib had lower sputum purulence scores than those taking placebo.

Limits of the analysis

Because this was a post hoc analysis, researchers looked back at trial data to answer a question the trial wasn’t originally designed to address. The sputum color categories were assigned after the fact, enzyme data were only available for part of the study population and self-reported color is subjective. The ASPEN trial was sponsored by Insmed, the maker of brensocatib.

Bringing sputum color into the conversation

Tracking sputum color is free, simple and can be done every day. For people with bronchiectasis, keeping note of changes and sharing them with a pulmonologist may add useful context to conversations about flare-ups and treatment. Any change in sputum color or symptoms should be discussed with a health care professional rather than used to make treatment decisions on your own.