Finding the right tests…
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Emergency services available 24 x 7
Finding the right tests…
Microbiology
Not every fluid sample pulled from around your lung needs testing for bacteria right away - sometimes the simpler question is whether the fluid is thin and watery or thick and inflamed. Pleural fluid routine and microscopy answers that by checking the sample's colour, counting red and white blood cells under the microscope, and measuring its protein against a blood sample taken at the same time. That protein comparison, called Light's criteria, sorts the fluid into transudate, usually from pressure-related causes like heart or liver failure, or exudate, which points toward inflammation, infection, or cancer. This test does not try to grow or name an organism - it describes the fluid and points to what gets tested next.
This test reports the fluid's appearance, a red and white blood cell count with the proportion of different white cell types, total protein, and sometimes glucose and LDH, along with a comparison against your blood protein and LDH levels. A mostly clear, low-protein fluid usually points to a pressure-related cause such as heart failure. A cloudy, high-protein fluid with many white blood cells points toward inflammation or infection and usually leads to further tests, including a culture, on the same sample or a repeat tap.
This test does not try to grow anything. A technician examines the fluid's colour by eye, then looks at it under a microscope to count red blood cells, white blood cells, and note which type of white cell, neutrophils or lymphocytes, dominates. Blood-tinged fluid can suggest trauma or cancer, while a cloudy fluid points toward active inflammation or infection.
The same sample's protein and LDH levels are measured and compared with a blood sample taken around the same time, using a method called Light's criteria. This comparison sorts pleural fluid into transudate, which usually reflects pressure or fluid overload from conditions like heart failure, kidney disease, or liver disease, or exudate, which points toward local inflammation, infection, or cancer and needs more specific tests.
A high count dominated by neutrophils suggests a bacterial process, often linked to pneumonia, while a lymphocyte-dominated count raises suspicion for tuberculosis or cancer instead. This distinction helps your doctor decide which follow-up tests are worth sending from the remaining fluid.
Routine and microscopy sorts and describes the fluid; it does not identify an infecting organism or look for malignant cells on its own. A separate pleural fluid culture is needed to name a bacterium or fungus, and a separate cytology exam is needed to look specifically for cancer cells.
Light's criteria compares protein and LDH levels in your pleural fluid against your blood to sort the fluid into transudate or exudate. Transudate usually points to a pressure-related cause like heart failure, while exudate points toward inflammation, infection, or cancer, which changes what gets tested next.
It can suggest infection through a high white blood cell count, especially one dominated by neutrophils, but it does not name the organism causing it. A separate pleural fluid culture is needed to identify a specific bacterium or fungus.
This test describes the fluid's appearance, cell counts, and chemistry without trying to grow anything. A pleural fluid culture is a separate test on the same or a repeat sample that specifically tries to grow bacteria or fungi to identify what's causing an infection.
This page is general information about the test, not medical advice. Your own report's reference ranges apply — please discuss your results with your doctor.
A high count, especially one dominated by neutrophils, usually points toward a bacterial infection or inflammation nearby, often linked to pneumonia. A count dominated by lymphocytes instead raises suspicion for tuberculosis or cancer, which needs different follow-up tests.
Not directly. This test can flag a fluid pattern, such as a lymphocyte-dominated exudate, that raises suspicion for cancer, but a separate cytology exam is needed to actually look for malignant cells under the microscope.
Comparing your pleural fluid's protein and LDH against a blood sample taken around the same time is what makes Light's criteria possible. Without a matching blood sample, the lab cannot sort the fluid into transudate or exudate.