A pleural fluid Gram stain is a laboratory test performed on fluid extracted from the area in the chest cavity outside of the lung. This area typically contains about 20 milliliters of clear or yellow fluid. Additional fluid can signal the presence of an infection around the lungs.
A surgeon obtains a sample of the fluid by inserting a hollow needle or a catheter between the ribs and into the chest cavity. This process is called thoracentesis. Lab technicians mix the sample with a bright violet stain known as the Gram stain.
The sample is then studied under a microscope to see how it interacts with the stain. The results give the medical team a good idea of which type of bacteria is causing the infection and which antibiotics would be most useful in its treatment.
The Gram stain is used to help identify different types of bacteria by how they accept Gram stain (i.e., Gram-positive or Gram-negative). It is one of several techniques laboratories use to identify exactly which type of bacteria is causing an infection.
A pleural fluid sample is obtained through a process called thoracentesis. A surgeon in a hospital or outpatient surgery setting handles thoracentesis. Doctors will take X-rays and ultrasounds of your chest prior to performing thoracentesis to pinpoint exactly where the extra fluid is located. They will also make sure that you do not have a blood-clotting problem.
Once you are cleared for the procedure, you will be asked to remove your clothes and will be given a hospital gown to wear. A technician will then direct you to sit on the edge of your bed or a chair. You will lean forward so that your arms and your head rest on a table in front of you. This gives the surgeon the best access to your chest. It is important to hold still while thoracentesis is being performed.
The technician will cleanse the skin over your ribs with an antiseptic solution once you are in the correct position. This won’t hurt, but it may feel cold. Your surgeon then will give you an injection of local anesthesia to numb your skin. These injections sting, but only briefly. The anesthesia will almost instantly numb the site of the injection and surrounding tissue.
The surgeon will insert a large needle or a catheter between your ribs into the pleural area when you are numb. Many people prefer to close their eyes and not watch this part of the procedure. Your doctor, technician, or nurse will ask that you to try not to cough during the procedure, even though you may want to. You may also be asked to breathe deeply or to move in a particular way to make the procedure easier.
After the needle is removed, a doctor or nurse will hold pressure on the wound to control any bleeding. A nurse will give you a bandage or place a small surgical dressing on the puncture once the bleeding has stopped. You will stay in the hospital or surgery center until your pulse, breathing, and blood pressure have stabilized.
Depending on the amount of pleural fluid that was obtained or that your doctor finds in your body, either the sample will be sent to the laboratory and you will be sent home, or a drainage tube may be attached to the needle and drain into a bottle or other type of collection system. If that is the case, you may be required to be admitted to a hospital.
Unless your doctor tells you differently, you can resume all of your normal activities once you are discharged.
Samples of your pleural fluid will be taken to a laboratory where they will be mixed with a violet-shaded stain called the Gram stain. If bacteria are present, the doctor studies how they react to the Gram stain by watching the color, number, structure, and shape of the cells.
Because the results of a Gram stain test can be complex and hard to interpret, laboratory professionals may need to spend several minutes studying the sample before reaching any decisions about diagnosis or treatment (Barenfanger et al., 2001).
A normal reading shows no bacteria and is reported as “no organisms seen.”
If the stain is Gram-positive, the bacterial cells turn purple or blue. Two well-known Gram-positive types of infection are staph (Staphylococci) and strep (Streptococci).
If the stain is Gram-negative, the bacterial cells turn red or pink. The most often seen Gram-negative cells are Moraxella catarrhalis, organisms of the diplococcus family, which cause infections such as bronchitis, sinusitis, and laryngitis. They are also associated with exacerbations of chronic obstructive pulmonary disease (COPD) and pneumonia.
You are unlikely to experience any negative side effects from thoracentesis. According to the Cleveland Clinic, the most common side effect, which happens to about 10 percent of patients, is a pneumothorax, a collapse of a part of the lung (Cleveland Clinic, 2006). This side effect is rarely serious enough to require medical attention, and usually fixes itself. However, you should contact your doctor if you experience extreme shortness of breath or heavy bleeding at the puncture site.
Your doctor will contact you about the results of your pleural fluid Gram stain within a few days. If the Gram stain confirms that a bacterial infection is causing your symptoms, your doctor will probably suggest a round of antibiotics.
In addition to doing the Gram stain, your doctor will use the pleural fluid for other tests such as a bacterial culture. These additional tests take longer to run, but are more sensitive when it comes to identifying the best antibiotic to treat a certain kind of bacteria. Therefore, your doctor may change your medication when he or she has seen the results of other tests.