What is the role of CSF gram staining in the workup of Haemophilus influenzae type b (Hib) meningitis?

Updated: Jul 09, 2018
  • Author: Prateek Lohia, MD, MHA; Chief Editor: Niranjan N Singh, MBBS, MD, DM, FAHS, FAANEM  more...
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Answer

Gram staining of the CSF may reveal the Hib pleomorphic gram-negative coccobacilli. If the CSF is cloudy, the stain should be performed on fresh uncentrifuged CSF. If the CSF is clear, it should be performed on the pellet of centrifuged CSF.

The probability of visualizing bacteria depends on the concentration of bacteria in CSF. Bacteria are identifiable in 60-90% of all cases of acute bacterial meningitis, particularly in cases where more than 105 CFU/mL are present. The specificity of a positive Gram stain for bacterial meningitis is approximately 95%. Gram staining does not provide a definitive identification of the bacteria and does not, of course, provide information concerning antibiotic sensitivities.

Oral or IV pretreatment with antibiotics may disable the identification of organisms by Gram stain. Indeed, the presence of organisms on Gram stain 24 hours after IV treatment has been initiated may be an important indicator of treatment failure.


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