Worried parent holding a feverish baby surrounded by symbolic blood cells and bacteria.

Decoding Fever: Are Routine Blood Tests in Infants Really Necessary?

Reese Marlowe in Health & Wellness November 2025 • 4 min read.

"A new study casts doubt on the reliability of standard blood cell counts for detecting serious infections in babies, prompting a re-think of how we approach fever."

Few things are more unsettling for parents than a fever in a young infant. It triggers a cascade of concerns: Is it just a virus, or something more serious? This worry often leads to a trip to the emergency room and a battery of tests, including the ubiquitous complete blood cell count (CBC). But what if this standard practice isn't as reliable as we thought?

For years, doctors have used CBC parameters, such as white blood cell (WBC) count and absolute neutrophil count (ANC), to help determine if a young, febrile infant has a serious bacterial infection (IBI), like bacteremia (bacteria in the blood) or bacterial meningitis. These infections, while rare, can be life-threatening, making early detection crucial. However, a recent study published in JAMA Pediatrics is shaking up this conventional wisdom.

The study, a planned secondary analysis of a large prospective observational cohort study, challenges the accuracy of relying solely on CBC results to identify infants at risk for IBIs. This is especially relevant in the era of widespread pneumococcal conjugate vaccines, which have altered the landscape of common childhood infections.

Why Are Routine Blood Cell Counts Not Always Reliable in Infants?

Worried parent holding a feverish baby surrounded by symbolic blood cells and bacteria.

The JAMA Pediatrics study, conducted across 26 emergency departments in the Pediatric Emergency Care Applied Research Network, analyzed data from 4313 febrile infants under 60 days old. The researchers looked at the accuracy of WBC count, ANC, and platelet count in identifying the 97 infants (2.2%) who had IBIs.

The results were surprising and concerning. The study found that common CBC thresholds had low sensitivity for detecting IBIs. In plain terms, a significant number of infants with serious infections had normal CBC results, potentially leading to delayed diagnosis and treatment. Here's a breakdown of the key findings:

White blood cell count less than 5000/μL: Missed 90% of infants with IBIs.
White blood cell count ≥15 000/μL: Only detected 27% of infants with IBIs.
Absolute neutrophil count ≥10 000/μL: Identified just 18% of infants with IBIs.
Platelets <100 x103 /μL: Picked up only 7% of infants with IBIs.

Even when the researchers used receiver operating characteristic (ROC) curves to determine optimal thresholds for CBC parameters, the accuracy remained poor. This suggests that relying solely on these numbers to rule out serious infection is a risky practice.

What Does This Mean for Parents and Doctors?

The study underscores the need for a more nuanced approach to evaluating febrile infants. While CBCs can provide some information, they shouldn't be the sole determinant in deciding whether a baby needs antibiotics or hospitalization. Doctors need to consider the infant's overall clinical picture, including their appearance, behavior, and other risk factors. Newer, more accurate diagnostic tools, such as procalcitonin and C-reactive protein, may also play a role in risk stratification, although these aren't yet universally available. As always, a parent's intuition counts for a lot, too; if you are worried, seek medical attention.

About this Article -

This article was crafted using a human-AI hybrid and collaborative approach. AI assisted our team with initial drafting, research insights, identifying key questions, and image generation. Our human editors guided topic selection, defined the angle, structured the content, ensured factual accuracy and relevance, refined the tone, and conducted thorough editing to deliver helpful, high-quality information.See our About page for more information.

This article is based on research published under:

DOI-LINK: 10.1001/jamapediatrics.2017.2927, Alternate LINK

Title: Accuracy Of Complete Blood Cell Counts To Identify Febrile Infants 60 Days Or Younger With Invasive Bacterial Infections

Subject: Pediatrics, Perinatology and Child Health

Journal: JAMA Pediatrics

Publisher: American Medical Association (AMA)

Authors: Andrea T. Cruz, Prashant Mahajan, Bema K. Bonsu, Jonathan E. Bennett, Deborah A. Levine, Elizabeth R. Alpern, Lise E. Nigrovic, Shireen M. Atabaki, Daniel M. Cohen, John M. Vanburen, Octavio Ramilo, Nathan Kuppermann

Published: 2017-11-06

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Everything You Need To Know

1

Why are complete blood cell counts (CBCs) traditionally used in infants with fever?

A complete blood cell count (CBC) traditionally helps doctors assess febrile infants for serious bacterial infections (IBIs) like bacteremia or bacterial meningitis, by evaluating parameters such as white blood cell (WBC) count and absolute neutrophil count (ANC). However, recent research indicates that relying solely on these CBC parameters may not be reliable due to low sensitivity in detecting IBIs. This means that many infants with serious infections can have normal CBC results.

2

According to the JAMA Pediatrics study, how reliable are routine blood cell counts in detecting serious infections in babies?

The *JAMA Pediatrics* study revealed that common CBC thresholds have low sensitivity for detecting IBIs in infants. For example, a white blood cell count less than 5000/μL missed 90% of infants with IBIs, and an absolute neutrophil count ≥10 000/μL identified only 18% of infants with IBIs. This suggests that relying solely on these CBC numbers to rule out serious infection is a risky practice.

3

What should doctors consider when evaluating febrile infants, given the findings of the JAMA Pediatrics study?

The study emphasizes that doctors should adopt a more comprehensive approach when evaluating febrile infants, considering not only CBC results but also the infant’s overall clinical condition, behavior, and other risk factors. Newer diagnostic tools like procalcitonin and C-reactive protein may also be useful for risk assessment, though they are not yet universally available. Parental intuition is also valuable and should prompt seeking medical advice when concerns arise.

4

What specific blood cell count parameters are being questioned in their ability to detect serious bacterial infections (IBIs) in infants?

The study specifically challenges the accuracy of white blood cell (WBC) count, absolute neutrophil count (ANC), and platelet count as sole indicators of serious bacterial infections (IBIs) in febrile infants. While these parameters have been traditionally used to detect infections like bacteremia or bacterial meningitis, the study found that many infants with IBIs had normal levels of these counts, highlighting their limited reliability. The low sensitivity of these markers suggests that relying on them alone can lead to missed diagnoses and delayed treatment.

5

What are the potential implications of the JAMA Pediatrics study on how doctors manage febrile infants?

The implications of the *JAMA Pediatrics* study could lead to a shift in how febrile infants are evaluated and treated. Instead of relying primarily on complete blood cell count (CBC) parameters, doctors may increasingly use a combination of clinical assessment, newer diagnostic tools like procalcitonin and C-reactive protein, and parental input to make more informed decisions about antibiotic use and hospitalization. This comprehensive approach aims to reduce the risk of missing serious bacterial infections (IBIs) while also avoiding unnecessary interventions for infants with viral illnesses.