Is it necessary?

Yes if under 28 days and febrile.

Older infants who otherwise appear well and have a positive urine dipstick or microscopy to suggest UTI, do not routinely require a lumbar puncture to exclude bacterial meningitis – PECARN study of 697 febrile infants aged 28-60 days with suspected UTI found no cases of bacterial meningitis.

In bronchiolitis, wide variation in practice – NICE bronchiolitis guidelines do not comment, and NICE suspected sepsis guidelines do not say that bronchiolitis exempts you from “suspected sepsis”.

Traumatic tap

Increasing RBC counts were statistically associated with increasing WBC counts (P < .001). But in febrile babies under 90 days,where RBC < 10 000/mm³ no real impact and reference range for WBC in uninfected infants with traumatic lumbar punctures was still 0 to 16/mm³.

CSF protein increased linearly with increasing CSF RBCs (up 1.1 mg/dL for every 1000 RBC).

Correct 500:1?  Sounds good in theory, but not in practice.  Predicted leukocytes matched observed leukocytes poorly for 682 CSF specimens.  Adjusted blood counts in CSF have no advantage over uncorrected counts for predicting bacterial meningitis. [PIDJ 2006;25(1):8-11DOI: 10.1097/01.inf.0000195624.34981.36 · ]