What condition is indicated by an abnormal finding of yellow sputum?

The presence of yellow sputum typically indicates an infection, with bacterial infections being the most common cause. When the body's immune system responds to a bacterial infection in the respiratory tract, it sends white blood cells to the site of infection to fight off the bacteria. As these immune cells accumulate and interact with the bacteria, they can lead to a change in the color of the sputum, resulting in a yellow or greenish hue.

In contrast, healthy lungs would not produce any sputum, or the sputum would be clear and thin. Chronic lung diseases may lead to various sputum colors depending on the condition and its progression, but yellow sputum is particularly characteristic of acute infections rather than chronic ones. Viral infections can also cause sputum production, often resulting in clear or even white sputum rather than the distinct yellow associated with bacterial infections.