Why is it necessary to do a spinal tap when you suspect a bacterial infection of the central nervous system? why can't you use a blood test?

Quite simple really ... The CNS actually doesn't receive as much nutrients or key components from the blood ... The blood actually dumps it into the Cerebrospinal fluid. The Cerebrospinal fluid therefore is directly in contact with the brain and if you want to find an infection you take from a source as close to thing as you can.