Perform an LP and analyze the results to dx bacterial meningitis

Roughly 20% of patients who receive lumbar punctures experience postdural puncture headaches (PDPH, also commonly called a ìspinal headacheî). These can be quite debilitating, sometimes even requiring hospitalization. The use of small gauge atraumatic needles and reinsertion of the stylet before needle removal reduces the risk of PDPH while immobilization does not. In addition to typical serologies, cerebrospinal fluid (CSF) gram stain and culture, the following CSF markers can aid significantly in the diagnosis of bacterial meningitis in adults: CSF blood glucose ratio ≤ 0.4, CSF WBC count ≥ 500/µL, and a CSF lactate ≥ 31.53 mg/dL.1

The Data
The data at hand comes from the rational clinical exam series in JAMA. The authors used fairly rigorous criteria to generate their summative conclusions. To answer the two central questions, they selected only randomized controlled trials with predominately adult patients (age > 18). In terms of minimizing the risk of PDPH, five trials with 587 patients met inclusion criteria looking at needle design and replacement of the stylet before needle removal.  Pertinent findings were as follows:

Reinsertion of the stylet before needle removal was associated with a statistically significant decrease in the rate of headache (NNT = 9) 

 A non-significant difference in the occurrence of PDPH was found when an ëatraumaticí needle was used versus a standard cutting one (NNT = 8).  Atraumatic needles are commonly used by anesthesiologists.  They are designed with a pencil tipped end as compared to the sharp beveled end of a cutting needle (this is the typical needle provided in an ED LP tray).  It should be noted that these findings may be limited by sample size and the exclusion of large anesthesia trials that show a significant reduction in risk when atraumatic needles are used.3-5

Interestingly, several factors often touted to impact the development of PDPH were not found to be statistically significant:
Bedrest
Number of LP attempts
Administration of oral or intravenous
fluids
Patient positioning
Clinician experience

Four studies met inclusion criteria for the predictive ability of CSF markers for the diagnosis of bacterial meningitis. Three biochemical markers were shown to have statistically significant predictive value (see table 1).

 

Table 1: Biochemical Markers with statistically significant predictive values

CSF-blood glucose ratio less than or equal to 0.4 (LR+=18; 95% CI: 12 to 27;
LR-=0.31; 95% CI: 0.21 to 0.45)

CSF white blood cell count (WBC) equal to or greater than 500/µL
(LR+=15, 95% CI: 10 to 22; LR-=0.30; 95% CI: 0.20 to 0.40.)

CSF lactate greater than or equal to 31.53 mg/dL (≥ 3.5 mmol/L; LR+=21; 95%
CI: 14 to 32; LR-=0.12; 95% CI: 0.07 to 0.23).

The Caveats
So why are cutting needles (Quincke) the standard for ED LP trays?  It could be that these needles are fractionally more expensive.  It is more likely that cutting needles are used because they always have been.  For more information on atraumatic needles in prepackaged LP kits, you can explore the following websites:
www.bd.com/anesthesia
bbraunusa.com/spinals

The Outcome
After reviewing the evidence, you find solace in the fact the clinical experience will probably not affect the development of procedural complications and champion the studentís fervor to practice ìreal emergency medicineî.  The key teaching points should reinforce the use of a small gauge spinal needle (preferably 22 gauge but no larger than 20 gauge) that is designed to be atraumatic (e.g. Sprotte or Whitacre needles).  He should also be cautioned to replace the stylet before needle removal.  In terms of CSF serologies, the standard lab set of cell count, gram stain, culture, protein, and glucose should be augmented with lactate (if available and cost effective in your ED).