Complete Blood Count (CBC) Test: What It Is, Why It's Done, and How to Prepare

The complete blood count, commonly called a CBC, is one of the most frequently ordered blood tests. It measures the cells that make up your blood, including red blood cells, white blood cells, and platelets, along with related values such as hemoglobin and hematocrit, giving your healthcare provider a broad picture of your overall health.

The Pericardial Fluid Aerobic Culture is a laboratory test that detects and identifies bacteria in the fluid collected from the pericardium (the sac surrounding the heart). The pericardial fluid is collected through a procedure called pericardiocentesis, where a needle is inserted into the pericardial sac to withdraw fluid. This fluid is then placed in special culture media that allows aerobic (oxygen-requiring) bacteria to grow. Once bacteria grow, they can be identified through various laboratory methods, and sensitivity testing can determine which antibiotics will be most effective in treating the infection.

This test is performed for many reasons. Your provider may order a CBC as part of a routine checkup, to help diagnose conditions such as anemia, infections, or blood disorders, to monitor an ongoing illness or treatment such as chemotherapy, or to investigate symptoms like fatigue, weakness, fever, bruising, or unexplained bleeding.

Your doctor may order this test if you have symptoms suggesting pericarditis or a pericardial effusion (fluid accumulation around the heart). These symptoms may include sharp chest pain that worsens with breathing or lying down, shortness of breath, fever, fatigue, and a dry cough. The test is crucial for determining whether a bacterial infection is causing your symptoms, which directly influences treatment decisions. Without proper diagnosis, bacterial pericarditis can lead to serious complications including cardiac tamponade (compression of the heart by accumulated fluid) or constrictive pericarditis.

The sample is collected through a simple blood draw. A healthcare professional will place a band around your upper arm, clean the skin at the inside of your elbow, and insert a small needle into a vein to collect blood into one or more tubes. The process usually takes only a few minutes, and you can return to your normal activities right afterward.

The sample for this test is pericardial fluid, not blood serum. Pericardial fluid is obtained through a procedure called pericardiocentesis, performed by a cardiologist or interventional radiologist. During this procedure, you will lie on your back, and the doctor will clean your chest area with antiseptic solution. Using ultrasound guidance for safety, a thin needle is inserted through the chest wall into the pericardial sac to withdraw fluid. The procedure typically takes 15-30 minutes. You may receive local anesthesia to minimize discomfort. After the procedure, you will be monitored for any complications such as bleeding or infection.

Your results are compared with established reference ranges. A low red blood cell count or low hemoglobin may point to anemia, while a high white blood cell count can suggest infection or inflammation. Platelet values that are too high or too low may affect clotting. Abnormal results do not always mean a serious problem, so your provider will interpret them together with your symptoms, medical history, and other tests.

A negative result means no aerobic bacteria were detected in your pericardial fluid culture. This could indicate that your condition is not caused by a bacterial infection—it might be viral, autoimmune, or due to other causes. A positive result indicates the presence of aerobic bacteria in the pericardial fluid, confirming a bacterial infection of the pericardium. The laboratory will identify the specific type of bacteria and perform sensitivity testing to determine which antibiotics will be most effective. Common bacteria that may be found include Staphylococcus aureus, Streptococcus species, and various gram-negative bacteria. Your doctor will use these results to prescribe the most appropriate antibiotic treatment.

A CBC is very safe. The most common risks are minor, such as brief pain or bruising at the needle site, and rarely lightheadedness during the draw. As for limitations, a CBC alone cannot diagnose a specific disease; abnormal values often need follow-up testing, and factors like dehydration, recent exercise, pregnancy, or certain medicines can temporarily influence your numbers.

The main risks associated with pericardiocentesis include bleeding, infection, puncturing the heart or lungs, and rarely, cardiac tamponade. These risks are minimized by using ultrasound guidance and having an experienced physician perform the procedure. Limitations of the culture include: (1) it only detects aerobic bacteria—anaerobic bacteria require separate testing, (2) some bacteria are difficult to grow in standard culture media, (3) prior antibiotic use can lead to false-negative results, and (4) the test takes several days to complete as bacteria need time to grow. If your doctor suspects anaerobic infection or fungal infection, additional tests may be ordered.