Colorless sputum can be a non-specific sign of many lung diseases, most of them not cancer; a large number of them being due to infection, allergy, chronic bronchitis or a condition called bronchiectasis. It can also be a symptom of heart disease.
Although it is a non-specific sign, it is not one that can be ignored, without risking your health. You should see a doctor and have a chest x-ray done, without delay. There should also be a careful examination of your sputum, for bacteria and especially for tuberculosis (TB) and atypical tuberculosis called nontuberculous mycobacterium (NTM) a common 1 being mycobacterium avium complex (MAC). All in this group require special cultures and will not grow on a routine sputum culture.
You may have to see a lung specialist, also known as a pulmonologist, to work this out, but whatever you do, definitely seek medical help.
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