Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (116 page)

   Immunocompromised state

   Papilledema

   Abnormal level of consciousness

   Focal neurologic abnormalities

   Primary testing usually includes aerobic bacterial culture, Gram stain, and CSF concentration of protein and glucose. The opening pressure should be measured at the time of lumbar puncture.

   Blood cultures, CBC, and other basic metabolic tests should be undertaken for initial evaluation of all patients with suspected ABM.

   CBC often shows changes related to acute infection (e.g., increased number of band forms, toxic granulations, Döhle bodies, vacuolization of PMNs).

   ESR, CRP, or other tests may indicate an intense inflammatory response.

   Infection may result in significant metabolic dysregulation.

   Gram stain is positive for the infecting organism in 25% of patients when organisms were present at 10
³
cfu/mL; sensitivity increased to 97% when organisms were present at 10
⁵
cfu/mL. The sensitivity of detection organisms by culture and Gram stain is improved by concentrating CSF, usually by centrifugation.

   The sensitivity of Gram stain depends on the infecting organism. Gram stain is positive in 90% of cases caused by staphylococci and pneumococci, 85% of cases caused by
H. influenzae
, 75% of cases caused by
N. meningitidis
, but only 30–50% of cases caused by gram-negative enteric bacilli. If antibiotics have been given before CSF obtained, Gram stain may be negative.

   Acridine orange staining may provide slightly greater sensitivity for detection of faintly staining organisms, but the technique requires use of a fluorescent microscope and technologist experience for smear interpretation.

   Testing for specific bacterial antigens may be used for rapid diagnosis in ABM. Kits are commercially available for detection of bacterial cell wall or capsular polysaccharide antigens of
H. influenzae
type b;
N. meningitidis
serogroups A, B, C, Y, and W135;
Streptococcus
, group B; and
S. pneumoniae.
Although tests show acceptable sensitivity and specificity, clinical studies suggest that the results of bacterial antigen testing rarely affect patient management; bacterial antigen testing is not recommended for routine evaluation of CSF in ABM.