Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (117 page)

   The most frequent and important differential diagnosis is between ABM and aseptic meningitis. The most useful test results are

   CSF identification of organism by stain or culture, specific nucleic acid, or antigen by PCR.

   Decreased CSF glucose or decreased CSF-to-serum glucose ratio if CSF glucose is normal.

   Increased CSF protein >1.72 mg/dL (1% of aseptic meningitis cases and 50% of ABM cases).

   CSF WBC >2,000/mm
³
in 38% of ABM cases and PMN >1,180/mm
³
but low counts do not rule out ABM.

   Peripheral WBC count is only useful if WBC (>27,200/mm
³
) and total PMN (>21,000/mm
³
) counts are very high, which occurs in relatively few patients; leukopenia is common in infants and elderly.

   CSF from patients with aseptic meningitis shows no organisms by Gram stain. WBC may be mildly elevated (<500 cells/mm
³
) with a lymphocyte predominance; protein may be moderately elevated; glucose level is usually normal.

   CSF from patients with ABM typically demonstrates markedly increased WBC count (>1,000 cells/mm
³
), with a PMN predominance, increased protein (>100 mg/dL), and decreased glucose (<50% of serum glucose concentration). Opening pressure is increased (normal 100–200 mm Hg)

   In 50% of cases caused by
L. monocytogenes
, Gram stain may be negative; the cellular response is usually monocytic, which may cause this meningitis to be mistaken for aseptic meningitis.

   Overall, CSF culture has a good sensitivity (70–92%) and high specificity (95%).

   A sufficient amount of CSF must be collected to allow the testing required. Priority must be given to rule out ABM and HSV, when suspected. Repeat sampling may be needed if initial testing is not informative. A minimum of 3–5 mL of CSF should be collected for diagnostic testing for mycobacteria or fungi.

   PCR methods have been developed for detection of some bacterial pathogens causing ABM, though FDA-approved methods are not available.