Interstitial lung disease (also called diffuse parenchymal disease) is a term used to describe a number of different disorders that affect the interstitial space. The interstitial space consists of the walls of the air sacs of the lungs (alveoli) and the spaces around blood vessels and small airways. Interstitial lung diseases result in abnormal accumulation of inflammatory cells in lung tissue, cause shortness of breath and cough, and have similarities in their appearance on imaging studies but are otherwise unrelated. Some of these diseases are very unusual.
Early in the course of these diseases, white blood cells White Blood Cells The main components of blood include Plasma Red blood cells White blood cells Platelets read more 
, macrophages, and protein-rich fluid accumulate in the interstitial space, causing inflammation. If the inflammation persists, scarring (fibrosis) may replace normal lung tissue. As alveoli are progressively destroyed, thick-walled cysts (called honeycombing because they resemble the cells of a beehive) are left in their place. The condition resulting from these changes is called pulmonary fibrosis.
Although the various interstitial lung diseases are separate and have different causes, they have some similar features. All lead to a decreased ability to transfer oxygen to the blood, and all cause stiffening and shrinkage of the lungs, which makes breathing difficult and causes cough. However, being able to eliminate carbon dioxide from the blood is usually not a problem.
Diagnosis of Interstitial Lung Diseases
Chest computed tomography
Pulmonary function testing
Arterial blood gas analysis
Because interstitial lung diseases cause symptoms that are similar to those of much more common disorders (for example, pneumonia Overview of Pneumonia Pneumonia is an infection of the small air sacs of the lungs (alveoli) and the tissues around them. Pneumonia is one of the most common causes of death worldwide. Often, pneumonia is the final... read more 
, chronic obstructive pulmonary disease Chronic Obstructive Pulmonary Disease (COPD) Chronic obstructive pulmonary disease is persistent narrowing (blocking, or obstruction) of the airways occurring with emphysema, chronic obstructive bronchitis, or both disorders. Cigarette... read more 
[COPD]), they may not be suspected at first. When an interstitial lung disease is suspected, diagnostic testing is done. Testing can vary by the disease suspected but tends to be similar.
Most people have a chest x-ray Chest Imaging Chest imaging studies include X-rays Computed tomography (CT) CT angiography Magnetic resonance imaging (MRI) read more , computed tomography Chest Imaging Chest imaging studies include X-rays Computed tomography (CT) CT angiography Magnetic resonance imaging (MRI) read more (CT) of the chest, pulmonary function tests Pulmonary Function Testing (PFT) Pulmonary function tests measure the lungs' capacity to hold air, to move air in and out, and to absorb oxygen. Pulmonary function tests are better at detecting the general type and severity... read more 
, and often arterial blood gas analysis Arterial Blood Gas (ABG) Analysis and Pulse Oximetry Both arterial blood gas testing and pulse oximetry measure the amount of oxygen in the blood, which helps determine how well the lungs are functioning. Arterial blood gas tests are invasive... read more . CT is more sensitive than chest x-ray and helps doctors make a more specific diagnosis. CT is done using techniques that maximize resolution (high-resolution CT). Pulmonary function tests often show that the volume of air that the lungs can hold is abnormally small. Arterial blood gas tests measure the levels of oxygen and carbon dioxide in the arterial blood and determine the acidity (pH) of the blood.
To confirm the diagnosis, doctors sometimes remove a small sample of lung tissue for microscopic examination (lung biopsy) using a procedure called fiberoptic bronchoscopy. A lung biopsy done this way is called transbronchial lung biopsy Procedures Done With Bronchoscopy 
. Many times, a larger tissue sample is needed and must be removed surgically, sometimes with use of a thoracoscope (a procedure called video-assisted thoracoscopic lung biopsy).
Blood tests may be done. They usually cannot confirm the diagnosis but are done as part of the search for other, similar disorders.
