Mycobacterial meningitis is most common in children between the ages of 6 months and 5 years and in the elderly. Chest x-ray film is reported to show hilar adenopathy in 50%-90% of children, but normal chest findings are more common in adults (in one series, about 50% of adults had normal chest x-ray findings). Purified protein derivative skin test result is negative in 5%-50% of patients. Mild to moderate anemia is frequent. The erythrocyte sedimentation rate is elevated in 80% of patients. CSF findings typically show moderate WBC elevation (usually <500/mm3 and almost always <1,000), with the majority being lymphocytes. However, there frequently are a significant number of neutrophils and sometimes, in the early stages, a majority of neutrophils. Protein level is usually mildly or moderately elevated (in one series, 76% had elevated protein levels on admission). Glucose level is decreased in 50%-85% of patients on admission. To have cell count, protein level, and glucose level all three normal on admission is extremely rare, although this happened in 3 of 21 patients in one report. Diagnosis is based on acid-fast smear, culture, exclusion of other etiologies, evidence of tuberculosis elsewhere, and clinical suspicion. Acid-fast smears on CSF are positive in about 20%-40% of cases (range, 3%-91%), and CSF culture is positive in only 37%-90% of cases. When findings are atypical, a nucleic acid probe with polymerase chain reaction (PCR) amplification on CSF can be helpful if it is available.