Hi I was wondering how a doctor diagnoses lung infections in individuals with weakened immune systems. I know many viruses and bacteria can invade the lungs but how do the doctors determine what treatment to use? I have had a phlem producing cough for the past week which I attribute to Bronchitis as I had a previous bout for 3 months before I got CFIDS. Now the zithromax is not helping this time. Should I get a sputum culture to see what is causing the illness?

There are a number of ways to diagnose what exactly is causing lung infection (pneumonia). After a careful exam, your doctor can order a chest X-ray. That won't specify the germ, but will document whether there is an abnormality consistent with pneumonia, since bronchitis doesn't cause abnormal X-rays. If you are producing sputum, it can be coughed into a sterile container and cultured. Sometimes, the germ will grow from your blood even if it doesn't grow from your sputum. Often the pattern of your symptoms and the exam have enough clues to help your doctor make an educated guess about what antibiotic will work; sometimes the antibiotic doesn't work because the germ is resistant, and sometimes it's the wrong antibiotic. If you are still sick with pneumonia and haven't responded to medicine, a diagnosis can also be made by bronchoscopy, a procedure that is done to obtain fluid and/or tissue from your lungs for culture and examination under the microscope.