This is a rapid, essential test to find the cause of inflammation or fluid buildup. It's a "cell census" for your body fluid (like spinal fluid or fluid from the lungs). Our lab precisely counts the total number of cells and then differentiates them (e.g., how many are neutrophils vs. lymphocytes). This count is a critical, fast clue: a high neutrophil count points to a bacterial infection, while high lymphocytes suggest a viral one. This test provides the immediate, actionable data your doctor needs to make a fast diagnosis.