Complete Blood Count (CBC) — What It Is, How It Is Done, and What Your Results Mean

What is this test? A complete blood count, or CBC, is a common blood test that measures the different cells in your blood, including red blood cells, white blood cells, and platelets. Your doctor may order it as part of a routine checkup or to help look into symptoms such as fatigue, fever, weakness, or unusual bruising and bleeding.

A pleural fluid bacterial culture is a laboratory test that detects and identifies bacteria in fluid collected from the pleural space—the area between your lungs and chest wall. Normally, this space contains a small amount of fluid that helps your lungs move smoothly when you breathe. When excess fluid accumulates (pleural effusion) or becomes infected, doctors need to determine if bacteria are present and what type.

How is the sample collected? A healthcare professional will draw a small amount of blood, usually from a vein in your arm, using a thin needle. The procedure takes only a few minutes. You may feel a brief sting when the needle goes in, and a small bandage will be placed over the puncture site afterward. No special preparation, such as fasting, is usually required.

The sample is collected through a procedure called thoracentesis. Your doctor will clean an area on your back or side, inject a local anesthetic to numb the area, and then insert a thin needle to withdraw fluid from the pleural space. The procedure usually takes 15-30 minutes. You may feel some pressure or discomfort during the needle insertion.

What happens in the laboratory? In the lab, your blood sample is placed in an automated analyzer that counts and measures each type of blood cell and reports their number, size, and other characteristics. If any value looks unusual, a laboratory specialist may prepare a stained blood smear and examine it under a microscope to confirm the findings before your results are released.

In the lab, the fluid sample is placed in special culture dishes and incubators that allow bacteria to grow. Technicians monitor the cultures over several days to see if bacteria appear. If bacteria grow, they are identified using additional tests, and the lab may perform antibiotic susceptibility testing (sensitivity testing) to determine which antibiotics will be most effective against the specific bacteria found.

What do the results mean? Your values are compared with reference ranges that reflect the levels typically seen in healthy people. Results outside these ranges can point to conditions such as anemia, infection, inflammation, or clotting problems, but an abnormal value alone does not confirm a diagnosis. Your doctor will interpret the numbers together with your symptoms, medical history, and other test results, so always review your report with your healthcare provider.

A positive result means bacteria were found in your pleural fluid, indicating a bacterial infection such as pneumonia, empyema, or tuberculous pleuritis. The report will identify the specific type of bacteria. A negative result could mean either no infection is present or that the infection is caused by organisms not easily detected by standard culture methods. Your doctor will interpret these results along with your symptoms, imaging studies, and other test results.