Pro tips for LPs in kids

Cite this article as:
Ben Lawton. Pro tips for LPs in kids, Don't Forget the Bubbles, 2015. Available at:

Though less commonly performed than it used to be, the lumbar puncture remains a key skill to master for anyone practising acute paediatrics. December’s Archives of Disease in Childhood Education and Practice contains an excellent paper entitled “How to use… lumbar puncture in children” (1), which flashes more pearls than a 4th of July garden party in the Hamptons. We share some of its wisdom below but highly recommend reading the paper to anyone who considers LP within their scope of practice.

How much is too much?

Adults have a CSF volume of about 150 mls and produce it at somewhere between 14-36 mls/hour. Neonates have about 50 mls of CSF, which they produce at a rate of 25 mls/day. Twenty drops of CSF equates to about 1 ml. How much CSF you need to take depends on what you want to do with it but 1.5 mls (or 30 drops) should be both safe and sufficient for your smallest patients.

It’s all about position

LP is commonly performed in the left lateral position in children. Hip flexion opens up the intervertebral spaces and makes the procedure easier. Neck flexion does nothing to help the procedure and will probably make it more uncomfortable for the child as well as making it harder for them to breathe. Supporting neonates in the sitting position with their hips flexed and their legs forward is associated with wider intervertebral spaces and less hypoxia than the left lateral position in this age group. Anecdotally I have recently changed my routine practice for neonatal LPs from left lateral to sitting and it also seems to be easier for holders with a wider range of experience to achieve an optimal position in relative comfort.

What am I aiming at?

The spinal cord in adults and older children ends around L1-L2, in neonates it extends down to L3. The sub-arachnoid space extends down to S2. L4-L5 is generally the best area to aim for (bearing in mind we are not always in the space we think we are) though L3-L4 is also OK. With the child in an appropriate position a line drawn between the most superior aspect of both Iliac crests (Tuffier’s line) crosses the midline over the body of L4 so the space just below this is ideal.

How deep do I need to go?

Medical folklore contains a few different answers to this question but the most scientific answer I have seen is following formula (2)

Depth (mm) = 0.4 x Weight (kg) + 20

So in a 10 kg child CSF should be found at a depth of 24 mm.

How can I make it more comfortable for the patient?

Use topical anaesthetic. EMLA has been shown to help in neonates(3). Post LP headache may be reduced by:

  • Using a smaller needle (25g in neonates, 22g in others)
  • Replacing stylet prior to needle withdrawal
  • Orientate the needle with the bevel parallel to the spine so it will separate the longitudinally running fibres of the Dura. (I think this feels natural in the left lateral position but requires more thought in the sitting position).

Have you thought about…

…CSF lactate? This is quite a good discriminator between viral and bacterial meningitis with levels over 3.5 suggestive of bacterial CNS infection. It’s not quite as accurate after antibiotic administration but may still be clinically useful.

…USS guidance? This is still waiting for a decisive trial in kids but small studies have shown it to be a promising option for further exploration.

Defence against the dark arts

Many a mythical formula has been conjured up to interpret a white cell count in the context of bloodstained CSF. The authors of this paper suggest you can get a feeling by comparing the ratio of white cells to red cells in the peripheral blood and basing your maths on this, but wisely acknowledge that accurate interpretation is difficult in this context. It’s also worth highlighting the well described trap that a normal CT does NOT exclude raised intracranial pressure.

Though LP is a procedure I perform fairly frequently this paper has shone a spotlight into several dusky areas of my knowledge and I hope it will suitably illuminate yours. Finally, if I am asked to supply a question for next year’s Christmas quiz it may well be “Where can you find Tuffier’s line?”.


  1. Schulga P, Grattan R, Napier C, et al. How to use… lumbar puncture in children. Arch Dis Child Educ Pract Ed 2015;100: 264–271.
  2. Bailie HC, Arthurs OJ, Murray MJ, et al. Weight-based determination of spinal canal depth for paediatric lumbar punctures. Arch Dis Child 2013;98:877–80.
  3. Kaur G, Gupta P, Kumar A. A randomized trial of eutectic mixture of local anesthetics during lumbar puncture in newborns. Arch Pediatr Adolesc Med 2003;157:1065–70.

Do antibiotics affect CSF results?

Cite this article as:
Tessa Davis. Do antibiotics affect CSF results?, Don't Forget the Bubbles, 2013. Available at:

Paediatricians often have to make a decision about whether to just go ahead and give antibiotics in suspected meningitis, or wait for a lumbar puncture (LP) – this could be due to parental refusal, an unstable patient, or a failed attempt.

There is often a discussion about repeating the LP later that day, or even the following day. We all know that having had antibiotics might affect the results. But what effect does it actually have?

Here, I summarise three key papers looking at this very question – do antibiotics affect cerebrospinal fluid (CSF) results in bacterial meningitis?

Paper 1 - Michael et al (2010)

View paper

Who were the patients?

Patients were adults from a large UK district hospital and were identified retrospectively through a coding diagnosis of meningitis.


How was bacterial meningitis defined for inclusion criteria?

Patients had to have clinical features consistent with meningitis and had to have had an LP with a cell count of >4 cells/ml.


How many patients were included?

They had 92 patients included in the study.

They had been diagnosed with meningitis and had an LP with >4 cells/ml.

They all received antibiotics prior to the LP.


What did they find?

What they concluded from the analysis was that once antibiotics have been started, an LP within 4 hours of antibiotic administration is still likely to be culture positive.  After the 4 hour mark the proportion of positive CSF cultures dwindled. 

Paper 2 - Kengaye et al (2001)

View paper

Who were the patients?

The cohort was drawn from all patients discharged from San Diego Children’s Hospital during a 4 year period.

The patient group was identified by a coding diagnosis of bacterial or suspected bacterial meningitis.


How was bacterial meningitis defined for inclusion criteria?

CSF culture positive with bacteria; CSF WCC >10/mm3 + CSF antigen or Gram stain positive; CSF WCC >100/mm3 + blood culture positive; or CSF WCC >4000/mm3 in the absence of positive cultures.


How many patients were included?

There were 128 patients included.

43% had an LP both pre- and post-antibiotics, 30% had antibiotics prior to LP, and 27% had LP prior to antibiotics.


What did they find?

There were far less positive CSF cultures in post-antibiotic LPs.

In particular N. meningitides was sterilized earlier than Strep. penumoniae or Group B Strep. meningitis.

No N. meningitides CSF cultures were positive by 2 hours post-antibiotics.

Their conclusion was that negative cultures occurred in 44% of post-antibiotic LPs and only 8% of pre-antibiotic LPs.  And that meningococcal meningitis is very quick to sterilize.

Paper 3 - Nigrovic et al (2008)

View paper

Who were the patients?

This was a retrospective cohort study across twenty Emergency Departments in US paediatric centres.

Paediatric patients were identified through a coding diagnosis of bacterial meningitis or unspecified meningitis; and a review of positive CSF cultures for bacteria.


How was bacterial meningitis defined for inclusion criteria?

CSF culture for positive for a bacterial pathogen; CSF WCC >=10 cells/microL with positive blood culture +/- positive CSF agglutination study results.


How many patients were included?

245 patients were included.

159 (65%) had the LP before antibiotic treatment and 85 (35%) had the LP after antibiotic treatment.

Of those who had received treatment prior to LP: 24% had oral antibiotics; 69% had IV antibiotics; 7% had both oral and IV antibiotics.


What did they find?

CSF culture results were significantly more likely to be negative after receiving antibiotics.

4 hours post-antibiotics: CSF WCC was not affected by the administration of antibiotics; but the CSF glucose was significantly higher; and the CSF protein lower (although not significantly).

This was more marked (and more significant) 12 hours post antibiotics.

What should we take from this for our daily practice?

I find it hard to draw any useful conclusions from the Benedict et al study. There are three major flaws with it:

  1. Every single patient had antibiotics before having their LP.  There is no comparison to the group that had the LP first (apparently there were none in this category) and so to draw any conclusion about the effect of the antibiotics on the CSF results seems a stretch.
  2. Patients were actually excluded if their CSF had <5 cells/ml and the culture was negative.  This seems to hugely skew the results.  It could be that there were thousands of (excluded) patients who had antibiotics prior to LP and that all their CSF sample showed no WCC and were culture negative.  This would vastly change the results.  It’s also in adults which makes it difficult to draw paediatric conclusions.
  3. The patients were split into viral and bacterial meningitis groups, and part of the way this decision was made was by looking at the CSF results.  It’s self-fulling spiral.

But, it is fair to say that in the patient group they looked at, the CSF cultures were still positive even after antibiotic administration as long as it was within 4 hours.  By the time there was an 8 hour gap post-antibiotics, none of the CSF cultures were positive.

All the studies were retrospective and relied on correct coding diagnosis.  The retrospective nature also made it difficult to accurately assess timing of lumbar puncture and antibiotics administration.  Deciding the inclusion criteria for bacterial meningitis in a study about the effect on CSF results is fraught with difficulties.

Kanageye’s paper, however, does indicate that CSF culture results are affected by antibiotic administration (even within a couple of hours) and so repeating the lumbar puncture the following day may well give false reassurance. And Nigrovic’s paper reinforces this finding, and adds that CSF glucose will increase, and CSF protein will decrease, post antibiotics (especially 12 hours post antibiotics).

Although the accuracy of the timing measurement is potentially flawed, this is something to bear in mind.  Often in paediatrics the LP is unsuccessful, the patient is treated anyway, and the LP will be repeated the following day.  This can give falsely reassuring results.  Be wary of making decisions around length or choice of antibiotic, based on a post-antibiotic lumbar puncture



Michael B, Menezes BF, Cunniffe J, Miller A, Kneen R, Francis G, Beeching NJ, Solomon T. Effect of delayed lumbar punctures on the diagnosis of acute bacterial meningitis in adults. Emerg Med J. 2010 Jun;27(6):433-8. 

Kanegaye JT, Soliemanzadeh P, Bradley JS. Lumbar puncture in pediatric bacterial meningitis: defining the time interval for recovery of cerebrospinal fluid pathogens after parenteral antibiotic pretreatment. Pediatrics. 2001 Nov;108(5):1169-74.

Nigrovic LE, Malley R, Macias CG, Kanegaye JT, Moro-Sutherland DM, Schremmer RD, Schwab SH, Agrawal D, Mansour KM, Bennett JE, Katsogridakis YL,Mohseni MM, Bulloch B, Steele DW, Kaplan RL, Herman MI, Bandyopadhyay S, Dayan P, Truong UT, Wang VJ, Bonsu BK, Chapman JL, Kuppermann N;American Academy of Pediatrics, Pediatric Emergency Medicine Collaborative Research Committee. Effect of antibiotic pretreatment on cerebrospinal fluid profiles of children with bacterial meningitis. Pediatrics. 2008 Oct;122(4):726-30.