Standard blood tests are rarely useful in detecting pulmonary alveolar proteinosis (PAP) and X-rays are a more common initial test. 

If you have an X-ray and the results are abnormal then your doctor will normally carry out more specialist lung (pulmonary) investigations. These may include:

• High-resolution CT scan of the lungs – this uses cross-sectional X-rays, which show parts of the body in layers.
• Pulmonary function studies – these look at various aspects of how your lungs work.  For example, a ‘lung volumes’ investigation looks at the size and volume of the lungs and a ‘gas transfer’ investigation measures the amount of oxygen that can be transferred from the lungs into the blood. 
• Bronchoscopy – this is an investigation where a doctor uses a ‘bronchoscope’ (a long flexible tube with a light at the end) to look at the inside of the breathing tubes and bronchi (branches of the airways).  
• Bronch-alveoloar lavage – this is often carried out at the same time as the bronchoscopy and involves passing a small amount of salty water through the bronchoscope and into the lung before sucking it out again.  The fluid that is sucked out is examined and is often striking because it looks very ‘milky’.
• Biopsy - in some cases a small biopsy sample is taken from just below the surface of the bronchi at the time of the bronchoscopy so that it can be examined in more detail.

In most cases of PAP, a CT scan is most useful for making a diagnosis.  However, other tests may be needed to confirm the diagnosis and make sure that the symptoms are not because of another illness.