When is a result not a result?

Hot on the heels of this great Archimedes on whether or not you should routinely do an LP in infants with a urinary tract infection, comes another publication, covered with a fairly critical review in Journal Watch.
What’s fascinating here is both “sides” drawing a conclusion that they can’t draw.
The authors took 392 infants aged 30 to 90 days who had had an LP, identified from a microbiology lab database, and found 57 who had evidence of a UTI.  They found that one of these 57 had disseminated sepsis.  They noted that this infant had other clinical features of sepsis, and conclude that routine LP is not needed in children aged 30 to 90 days who have UTI and no other features of sepsis.
Here’s where I think they go wrong:  They then go on to state that abnormal urinalysis has a negative predictive value for meningitis of 98.2%.  This might be arithmetically correct – I’ve not checked – but it is wrong to do.  The reason for this is that one of the groups contains just one patient, and you should be very careful of any statistical conclusion from just one patient.  What if this was the only patient from the next thousand?  Or what if it was the first of a series of ten patients about to turn up in the analysis?
In Journal Watch, the commentator concludes the reverse is true; that the children aged 30 to 90 days with UTI remain at high risk and so should have LP. However, if we assume that this study has underestimated the numbers of menigitis even fourfold, that’s still 53 LPs that were unnecessary.  Some of those – at some point – will cause harm.
Who should we believe?  The truth probably lies somewhere between the two, and may be articulated as follows:
– in children with UTI, the younger the child, the higher the risk of disseminated sepsis
– at very young ages, it is likely that a prescriptive rule of “always do an LP regardless of clinical signs” may be safest, but as children get older, clinical assessment should gain ascendency
– the conclusion from the Archimedes article, which is: “Between 0% and 2% of infants under the age of 3 months with urinary tract infection have co-existing bacterial meningitis” stands unchanged with this addition to the literature.

My clinical bottom line?  Think about doing an LP.  Think very carefully, and then follow your clinical judgement, based on the clinical progress of the child.

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