Does Height or Length of Fever Predict SBI in Children?
The duration (length) of fever is associated with increased risk of serious bacterial infection (SBI) in febrile infants, while the height of fever shows a modest but significant association; however, neither parameter alone is sufficient to rule out SBI and must be integrated with other clinical and laboratory findings.
Duration of Fever (Length)
Longer duration of fever is significantly associated with SBI risk in young febrile infants. 1
Infants aged 2-6 months with SBI had significantly longer mean fever duration (26.5 hours, SD 41.5) compared to those without SBI (18.6 hours, SD 21.7). 1
The American College of Emergency Physicians identifies longer duration of fever as one of the clinical variables associated with greater rates of SBI in infants aged 57-180 days with rectal temperatures ≥38.0°C. 1
Laboratory markers (WBC, ANC, CRP) become more predictive of SBI when fever duration exceeds 12 hours, with area under the curve values of 0.85,0.83, and 0.92 respectively for fever >12 hours versus 0.37,0.42, and 0.68 for fever ≤12 hours. 2
Children with prolonged fever ≥5 days have higher SBI incidence (8.4%) compared to those with fever <5 days (5.7%). 3
Height of Fever (Temperature)
The height of fever demonstrates a modest but consistent association with SBI risk, though it is NOT associated with increased risk according to some guideline-level evidence. 1
The Contradictory Evidence:
The American College of Emergency Physicians guideline explicitly states that "height of fever, sex, and age were not associated with increased risk of SBI" in their analysis of 429 infants aged 57-180 days. 1
However, more recent research demonstrates that height of fever is significantly associated with SBI risk, with an odds ratio of 1.5 (95% CI 1.2-1.8) in a large multicenter study of 4,821 febrile infants. 4
In a single-center study of 1,057 febrile infants ≤90 days, mean temperature was significantly higher in those with SBI (38.5°C vs 38.3°C, p<0.005), with adjusted odds ratio of 1.76 (95% CI 1.32-2.33) after controlling for age, gender, and illness severity. 5
Critical Clinical Limitations
Neither fever height nor duration can reliably exclude SBI when used in isolation. 1
10% of infants with SBI were classified as "not ill-appearing" based on Yale Observation Scale scores, and 3 of 4 cases of bacteremia appeared well on clinical examination. 1
15.5% of infants with SBI had initial triage temperatures ≤38°C, demonstrating that normal or low-grade fever does not exclude serious infection. 5
For temperature ≥39°C, sensitivity for SBI is only 15.5% with specificity of 90.4%, meaning most infants with high fever do not have SBI, but many with SBI do not have high fever. 5
Clinical appearance alone cannot reliably exclude serious bacterial infections, as only 58% of infants with bacteremia or bacterial meningitis appear clinically ill. 6
Practical Clinical Algorithm
When evaluating febrile infants, integrate fever characteristics with other risk factors:
Age <60 days: Requires comprehensive evaluation (blood culture, urine culture, inflammatory markers, lumbar puncture) and hospitalization with empirical antibiotics regardless of fever height or duration. 6, 7
Age 2-6 months with fever >12 hours: Laboratory markers (WBC, ANC, CRP) become more reliable predictors; obtain complete workup including urine testing. 2, 1
Fever ≥5 days: Higher SBI risk (8.4%); warrants careful clinical assessment, diagnostic workup, and consideration of non-infectious causes including Kawasaki disease. 3
Temperature ≥39°C: Increases likelihood of SBI but cannot be used alone; must be combined with clinical appearance, laboratory markers, and other risk factors. 4, 5
Additional Risk Factors to Consider
The following factors increase SBI risk independent of fever characteristics: 1
- Ill appearance (Yale Observation Scale score ≥21 associated with 40% SBI rate)
- Uncircumcised male infants (36% bacteriuria rate vs 1.6% in circumcised males)
- Absence of obvious viral source (18.1% SBI rate vs 6.1% with obvious source)
- Negative viral testing (13.5% SBI rate vs 4.9% with positive viral testing)
- Elevated inflammatory markers (WBC, ANC, CRP)
Key Clinical Pitfall
Never rely on recent antipyretic use to assess disease severity, as antipyretics can mask both fever and clinical signs of serious illness. 6, 8