A 2-year-old child has a fever, stiff neck, and is irritable. Gram stain smear of spinal fluid reveals gram-negative, small pleomorphic coccobacillary organisms. What is the most appropriate procedure to follow in order to reach an etiological diagnosis?

Meningitis caused by H. influenzae cannot be distinguished on clinical grounds from that caused by pneumococci or meningococci. The symptoms described are typical for all three organisms. H. influenzae is a small, gram-negative rod with a polysaccharide capsule. It is able to grow on laboratory media if two factors are added. Heme (factor X) and NAD (factor V) provide for energy production. Use of the conjugate vaccine (type b polysaccharide) reduces the disease incidence more than 90%. Pneumococci are gram-positive diplococci, and meningococci are gram-negative diplococci, which grow on blood agar and chocolate agar with no X and V factors needed, respectively. Salt-mannitol agar is used to distinguish S. aureus from other oral flora. Streptococci do not produce catalase while many other organisms do. Coagulase production is another test to identify S. aureus. Commercial kits are available for immunologic detection of H. influenzae antigens in spinal fluids, but currently none are available to measure specific antibody in CSF.