What finding on lumbar puncture indicates a diagnosis of bacterial meningitis?

A decreased glucose level in the cerebrospinal fluid (CSF) is a key diagnostic indicator of bacterial meningitis. In this condition, bacteria invade the central nervous system, leading to an inflammatory response that disrupts normal glucose metabolism. The bacteria consume glucose, and inflammatory cells may further impair glucose transport, resulting in lower glucose levels compared to serum glucose. Typically, the CSF glucose in bacterial meningitis is less than 40% of the serum glucose level, distinguishing it from other types of meningitis, such as viral meningitis, where glucose levels usually remain normal.

In contrast, increased lymphocytes are often more indicative of viral or atypical infections, normal protein levels would be more consistent with non-infectious causes or viral meningitis, and increased red blood cells could suggest a traumatic tap or hemorrhagic process rather than a specific indication of bacterial meningitis. Thus, the decreased glucose level is a crucial finding that points towards bacterial infection within the central nervous system.