Friday, 18 April 2014

Which blood test is the best at identifying serious bacterial infection (SBI) in febrile children without a source?

In the new era of widespread childhood immunization the rates of SBI in children has been plummeting. Hib and PCV7 vaccine have made our lives almost boring in the ED. When was the last time you saw a good case of pediatric epiglotitis? Have you even heard of buccal cellulitis? Well, at least vaccination has been good for kids.

But widespread immunization has also created a diagnostic problem for us. The prevalence of serious bacterial illness is now so low it is a kin to finding a needle in a haystack. Imperfect clinical acumen will certainly find more false positives than true positives. Or perhaps we will just get complacent and call everyone negative until it’s too late. We really don’t want to be missing something serious. I wish there was a blood test to help us.

Along comes a well done systematic review and meta-analysis published in the Annals of Emergency Medicine looking at the old favourites WBC, CRP and the new kid on the block procalcitonin. Which one is best at picking up SBI?

After a good systematic review, 8 studies were included for a total of 1,883 patients comparing the three blood tests. The primary outcome was the ability to detect SBI in children with fever without a source.

Right off the bat there is a problem with the definition of SBI. Included are some rare but serious conditions such as meningitis and sepsis. But also included are UTI, gastroenteritis, pneumonia and occult bacteremia. Cynics might proclaim, “If the bacteremia is so occult, then why should I care?” Do I really need a blood test to help me identify pneumonia, gastroenteritis or UTI’s? Shouldn’t I have picked up on this by other means? How many of the UTI’s were really false positive? As is the case in many of the studies of SBI, the majority had UTI as their “serious” illness.

In the end procalcitonin was the best of the worst. Its point estimate of sensitivity was 0.83 and specificity 0.69. Despite getting bigger numbers to crunch in this meta-analysis the confidence interval for sensitivity was as low at 0.70. CRP was hardly better. WBC was as good as flipping a coin. One might reasonably ask if this is good enough.

Another concern is these studies are performed in a heterogeneous population. These tests likely perform better at the more extreme spectrum of disease. I am not looking for a blood test to help me when it is quite clear one way or another if a child is sick. I want to know if it can help me when I’m on the fence. Therefore these tests may perform worse in the exact situation when I want to use them the most.

So what’s the take home point? At best these tests are a piece of the puzzle when evaluating the child without a source of fever. We should never use them in isolation or be overly reliant upon their ability to change our medical decision making. It is unusual for them sway my cognitive process in the ED. Believe it or not, I actually have the patient in front of me. Sometimes I can admit them or observe them for a period of time... crazy.

Covering:

Yo Hc, Hsieh PS, Lee SH, et al. Comparison of the Test Characteristics of Procalcitonin to C-Reactive Protein and Leukocytosis for the Detection of Serious Bacterial Infections in Children Presenting With Fever Without a Source: A Systematic Review and Meta-analysis. Ann Emerg Med 2012;60:591-600.

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