Bacterial Meningitis: This condition is characterized by an elevated white blood cell (WBC) count in the cerebrospinal fluid (CSF) and low glucose levels, which are indicative of bacterial infection. The presence of 18 WBCs and low glucose in the CSF strongly suggests bacterial meningitis as the primary diagnosis.

Tuberculous Meningitis: Although less common than bacterial meningitis, tuberculous meningitis can also present with elevated WBC counts and low glucose in the CSF. It is more likely to have a higher lymphocyte count and a more chronic presentation.
Fungal Meningitis: Fungal infections, such as cryptococcal meningitis, can cause similar CSF findings, especially in immunocompromised patients. The diagnosis might be considered if the patient has a relevant exposure history or is immunocompromised.
Viral Meningitis with Secondary Bacterial Infection: While viral meningitis typically presents with normal glucose levels, a secondary bacterial infection could lead to low glucose and elevated WBCs. However, the WBC count is usually higher in purely bacterial infections.

Subarachnoid Hemorrhage: Although not directly related to WBC count or glucose levels, subarachnoid hemorrhage can lead to a decrease in CSF glucose due to the metabolic demands of blood in the CSF. It's crucial to consider this diagnosis due to its high morbidity and mortality if missed.
Central Nervous System (CNS) Lymphoma: CNS lymphoma can cause elevated WBC counts and low glucose in the CSF, especially in immunocompromised patients. It's a critical diagnosis not to miss due to its severe prognosis if left untreated.
CNS Vasculitis: This condition can lead to inflammation within the CNS, potentially affecting CSF parameters. While less common, it's a diagnosis that could have significant implications if missed.

Parasitic Meningitis: Certain parasites, like Naegleria fowleri, can cause meningitis with abnormal CSF findings. However, these infections are rare and usually associated with specific exposure histories (e.g., swimming in contaminated water).
Sarcoidosis Involving the CNS: Sarcoidosis can rarely cause meningitis with elevated WBCs and altered glucose levels in the CSF. The diagnosis would be considered in patients with known sarcoidosis or those with systemic symptoms suggestive of the disease.
Leptomeningeal Carcinomatosis: Metastatic cancer involving the leptomeninges can lead to abnormal CSF findings, including low glucose and elevated WBCs. This diagnosis is rare but critical to identify due to its implications for patient management and prognosis.