Lumbar puncture is frequently performed in primary care. Properly interpreted tests can make cerebrospinal fluid (CSF) a key tool in the diagnosis of a variety of diseases. Proper evaluation of CSF depends on knowing which tests to order, normal ranges for the patient's age, and the test's limitations. Protein level, opening pressure, and CSF-to-serum glucose ratio vary with age. Xanthochromia is most often caused by the presence of blood, but several other conditions should be considered. The presence of blood can be a reliable predictor of subarachnoid hemorrhage but takes several hours to develop. The three-tube method, commonly used to rule out a central nervous system hemorrhage after a "traumatic tap," is not completely reliable. Red blood cells in CSF caused by a traumatic tap or a subarachnoid hemorrhage artificially increase the white blood cell count and protein level, thereby confounding the diagnosis. Diagnostic uncertainty can be decreased by using accepted corrective formulas. White blood cell differential may be misleading early in the course of meningitis, because more than 10 percent of cases with bacterial infection will have an initial lymphocytic predominance and viral meningitis may initially be dominated by neutrophils. Culture is the gold standard for determining the causative organism in meningitis. However, polymerase chain reaction is much faster and more sensitive in some circumstances. Latex agglutination, with high sensitivity but low specificity, may have a role in managing partially treated meningitis. To prove herpetic, cryptococcal, or tubercular infection, special staining techniques or collection methods may be required.