The pleural fluid gram stain is a test to diagnose bacterial infections in the lungs.

Pleural fluid is found in the space around the lungs. First, a sample of the pleural fluid is drained. This is done using a procedure called thoracentesis.

The health care provider cleanses an area on the chest with germ killing (antibacterial) soap and numbs it with local pain killing medicine (anesthetic). A needle placed between the ribs removes a sample of fluid from the chest.

The fluid sample is placed onto a microscope slide and mixed with a violet stain (called a gram stain). A laboratory specialist uses a microscope to look for bacteria on the slide. If bacteria are present, the color, number, and structure of the cells are used to identify the specific organism.

It is important not to cough, breathe deeply, or move when the fluid sample is being taken. There is no other special preparation for the test.

You may feel a stinging sensation when the anesthetic is injected. You may feel some pressure and slight pain in the area when the thoracentesis needle enters the pleural space. You may have a chest x-ray after the test to make sure the test did not affect the lung tissue.