Final answer:
In a child older than 28 days with a traumatic LP, one WBC is typically subtracted for every 500 to 1000 RBCs present to estimate the true CSF WBC count, though the exact ratio can vary and should be confirmed with local clinical guidelines or a hematologist.
Step-by-step explanation:
The question pertains to the correction factor for red blood cells (RBCs) to white blood cells (WBCs) in the context of a traumatic lumbar puncture (LP). When performing a lumbar puncture, trauma can cause peripheral blood contamination in the cerebrospinal fluid (CSF). This can result in an artificially elevated WBC count due to the presence of RBCs from the blood. To adjust for this, a correction factor is often used to estimate the true WBC count. For a child older than 28 days with a traumatic LP, the commonly used correction factor is subtracting one WBC for every 500 to 1000 RBCs present. However, it should be noted that the exact ratio might vary between institutions and according to the specific clinical situation. Therefore, it's important to consult local clinical guidelines or a hematologist for the appropriate correction factor.