The febrile infant (younger than 90 days of age): Definition of fever

INTRODUCTION — 

This topic will review the definition of fever in young infants younger than 90 days of age.

The evaluation and management of febrile young infants ≤90 days of age is discussed separately:

●(See "Neonatal bacterial sepsis: Clinical features and diagnosis in neonates born at or after 35 weeks gestation" and "Neonatal bacterial sepsis: Treatment, prevention, and outcome in neonates born at or after 35 weeks gestation".)

●(See "The febrile neonate (28 days of age or younger): Outpatient evaluation" and "The febrile infant (29 to 90 days of age): Management".)

●(See "The febrile infant (29 to 90 days of age): Outpatient evaluation" and "The febrile infant (29 to 90 days of age): Management".)

EPIDEMIOLOGY — 

Fever is a prominent symptom of many different disease processes. Based upon one large, population-based observational study, approximately 1.4 percent of infants younger than three months of age coming to the office setting, emergency department, or within 24 hours of birth present for evaluation of fever [1]. Neonates and young infants may manifest fever as the only sign of significant underlying infection. The incidence of invasive bacterial infection (IBI; bacteremia and/orbacterial meningitis) is higher in infants younger than three months of age, particularly those under 28 days, than at any other time in childhood [2-5]. In addition, these young patients can experience significant morbidity from some viral infections. (See "The febrile infant (29 to 90 days of age): Outpatient evaluation", section on 'Etiology' and "The febrile neonate (28 days of age or younger): Outpatient evaluation", section on 'Etiology'.)

DEFINITION OF FEVER — 

Rectal temperatures are the gold standard for detecting fever in infants ≤90 days of age. A rectal temperature ≥38°C (100.4°F) defines fever in these patients. We use electronic rectal thermometers with disposable covers. Glass thermometers should not be used because of difficulties with reading the temperature correctly, risk of injury to the patient if they break, and potential for spreading infectious disease.

This temperature is the one used by the majority of studies that have focused on the risk of serious infection in young infants and corresponds to a value that is two standard deviations above the mean in this age group [6].

APPROACH TO THE REPORT OF FEVER — 

The approach to a report of fever in young infants depends on how the temperature was taken. However, any infant ≤90 days old in whom fever is suspected should undergo a careful history, physical examination, and appropriate ancillary studies according to age and clinical findings if fever is confirmed or strongly suspected by caregiver report. (See "Neonatal bacterial sepsis: Clinical features and diagnosis in neonates born at or after 35 weeks gestation" and "The febrile neonate (28 days of age or younger): Outpatient evaluation" and "The febrile infant (29 to 90 days of age): Outpatient evaluation".)

Rectal temperature — Febrile young infants ≤90 days old with a fever confirmed by rectal temperature should undergo a careful evaluation for the source of the fever and the likelihood of invasive bacterial infection as well as serious viral infection according to age and clinical status. (See "Neonatal bacterial sepsis: Clinical features and diagnosis in neonates born at or after 35 weeks gestation" and "The febrile neonate (28 days of age or younger): Outpatient evaluation" and "The febrile infant (29 to 90 days of age): Outpatient evaluation".)

Other methods of temperature measurement — Axillary, temporal artery, and tympanic membrane temperatures appear less reliable for detecting fever in young infants when compared with rectal thermometry as the gold standard. Temperatures obtained by non-rectal routes should be confirmed by rectal temperature measurement except when rectal thermometry is contraindicated such as in patients with neutropenia, bleeding diathesis (eg, thrombocytopenia), or necrotizing enterocolitis. Whenever feasible, we obtain the rectal temperature of infants initially assessed using other means of temperature measurement. When the measurements are close in time (ie, within 15 to 30 minutes), then we use the rectal temperature to determine if a fever is present. If the measurements are not close in time, then if either measurement indicates a fever, we proceed with an evaluation according to the patient age and clinical findings. (See "The febrile infant (29 to 90 days of age): Outpatient evaluation".)

When rectal thermometry is contraindicated in a young infant, we use an axillary temperature to determine whether a fever is present. A temperature of 38°C (100.4°F) or greater by axillary measurement should be regarded as fever. Axillary temperatures that are near 38°C (100.4°F) should be interpreted on a case-by-case basis using the overall clinical assessment. Options include serial exams and observation or a laboratory evaluation. (See "Fever in infants and children: Pathophysiology and management", section on 'Temperature measurement'.)

Observational studies have established the following relationships between rectal temperature and other measurements of body temperature in infants and children as follows (see "Fever in infants and children: Pathophysiology and management", section on 'Temperature measurement'):

●Axillary temperature is most commonly performed as an alternative to rectal temperature in the young infant with contraindications to rectal measurement. It is consistently lower than rectal temperature, but the absolute difference in individual patients varies too widely for a standard conversion.

●Systematic reviews have concluded that infrared tympanic membrane thermometry shows insufficient agreement with established methods of core temperature measurement to be used in situations where detection of fever has important clinical implications, such as in febrile young infants.

●Infrared contact and noncontact forehead (temporal artery) thermometers also have contradictory results when compared with rectal temperatures and should not be used to determine the presence of fever in a young infant. Readings may be significantly greater or lower than rectal temperature and may be impacted by vascular changes in the skin such as vasodilation or vasoconstriction related to ambient temperature.

●Oral temperatures in young infants are unreliable. Based upon an observational study in 148 infants younger than three months of age, oral temperature measurement with a pacifier thermometer had a low sensitivity for detection of fever (48 percent) when compared with rectal temperature and a number of infants were unable or unwilling to suck on the device long enough to obtain a reading [7].

Because of the variability from device to device and by site of measurement, we do not suggest altering the reported temperature based on the device used for estimating the rectal temperature. For example, it is not advised to "add a degree" to axillary values but rather report the value with the method of measurement used. As with any measured data, the clinician must then use all clinical findings to interpret the results and to plan an appropriate evaluation.

Caregiver measurement — Young infants who are afebrile at presentation but have a caregiver report of fever taken by a reliable means at home still have a non-negligible risk for a serious bacterial infection [8-10]. Furthermore, the risk of an invasive bacterial illness such as bacteremia or meningitis does not appear to be significantly different in these patients when compared with febrile young infants. In addition, afebrile infants on presentation who were given antipyretics prior to arrival (thereby potentially masking fever but not changing the course of the underlying infectious process) remain at the same risk as infants who are febrile at presentation.

Our approach to caregiver report of fever depends upon how the temperature was taken as follows:

●Rectal temperature – Caregivers frequently report an elevated temperature in an infant who is afebrile at the time of evaluation. A history of objective fever by rectal temperature at home requires further evaluation according to age and clinical findings, even if the infant is afebrile at the time of presentation. (See "The febrile neonate (28 days of age or younger): Initial management", section on 'Approach' and "The febrile infant (29 to 90 days of age): Management", section on 'Management'.)

In one small observational study of 40 such patients presenting to an emergency department and performed prior to widespread availability of Haemophilus influenzae type b and pneumococcal conjugate vaccines, the risk of bacterial infection, including urinary tract infection, bacteremia, or meningitis was approximately 10 percent [11]. A more recent prospective cohort study performed after routine use of Haemophilus influenzae type b and heptavalent pneumococcal vaccines evaluated 835 infants with a documented fever by rectal temperature at home but who were afebrile when seen in a primary care office and found a risk of about 1 percent for bacteremia or meningitis [12]. Taken together, these studies suggest that the risk of bacterial infection is non-negligible in infants who are afebrile on presentation but had a fever documented by rectal temperature at home.

●Axillary temperature – Limited evidence suggests that the risk of invasive bacterial illness in infants whose caregivers report a fever based upon axillary measurements may be similar to infants who are febrile upon medical evaluation. Thus, we support further evaluation according to age and clinical findings in these patients. For example, in a retrospective study of almost 2500 infants with a caregiver history of fever (most taken by axillary measurement [personal communication, Dr. Santiago Mintegi]), the rate of IBI was not significantly different for afebrile infants based upon emergency department assessment when compared with febrile infants (IBI 2.4 percent for both) [8]. Notably, the local protocol for evaluation included lumbar puncture for all infants who were not well appearing as well as all infants <21 days of age.

●Other methods of temperature measurements – Evidence is lacking regarding the risk of IBI in young infants with fevers reported based upon less reliable means of temperature assessment such as contact or non-contact infrared temporal artery temperature, oral pacifier thermometer, or infrared tympanic membrane temperature.

We advise measurement of a rectal temperature in this situation. When parents or primary caregivers report an elevated temperature within the past 24 hours that is based upon other methods of measurement, our approach is to proceed with an evaluation according to the patient age and clinical findings regardless of the rectal temperature at clinical presentation or the timing between measurements by the caregiver and the medical provider. This approach is in part informed by an understanding that fever is not a static process. In these situations, varying temperature readings over time may reflect natural variability rather than a lack of fever.

Noncontact thermometers (infrared thermal scanners), which have become the preferred instruments from an infection control standpoint when screening patients for COVID-19, allow temperatures to be taken with minimal to no physical contact with the person being examined. There is no consensus as to the optimal cutoff temperature for determining the existence of a fever for any of these devices [13]. However, thermal screening has reasonable diagnostic accuracy for the detection of fever. For example, in one systematic review of 30 studies and meta-analysis of 11 studies, many of which used noncontact infrared thermometers (NCITs), negative predictive values for excluding fever were high and remained high (97.7 to 100 percent) over a wide range of prevalence (0.0001 to 10 percent) [14]. On the other hand, the sensitivity and specificity of NCITs and thermal scanners in the detection of fever ranged widely with high heterogeneity on meta-analysis likely due to differences in type and method of noncontact device used, varying reference fever thresholds and method of measurement, patient age, and setting between the studies.

●Tactile temperature – When a caregiver reports a tactile (subjective) fever and a rectal temperature shows no fever, our approach is follows:

•If the caregiver is calling to report a tactile fever, we ask the caregiver to verify with a rectal temperature at home. If the measured rectal temperature is less than 38°C, there is no documented fever.

•If the patient presents without a measured documented temperature of 38°C or greater, the infant may avoid further evaluation if all of the following conditions are met:

-No antipyretic medication (eg, acetaminophen) has been given.

-History does not suggest an increased risk for bacterial infection (eg, no perinatal risk factors such as prematurity, prolonged rupture of membranes, or maternal colonization with Group B streptococcus) and, by history, the infant has normal behavior and feeding and no associated difficulty breathing, apneic spells, or color changes.

-Appearance and physical examination are normal.

-Reliable follow-up within 12 to 24 hours can be arranged.

-Caregivers can monitor the rectal temperature at home, understand indications to seek medical attention, and have transportation.

For patients in whom reliable follow-up is not assured and who have not received antipyretic therapy, observation for lack of fever by rectal measurement at one or two additional time points in the department (eg, for one to two feeding cycles) remains an option. It is reassuring if the patient feeds well, remains afebrile, and continues to look well. This time can be used to continue to educate parents or other caregivers on supportive care and signs and symptoms to return.

Based upon one very small observational study performed prior to widespread availability of Haemophilus influenzae type b and heptavalent pneumococcal vaccines, only 1 of 26 patients with tactile fever at home who were afebrile in the emergency department had a serious bacterial illness (urinary tract infection) [11]. The fact that these patients differ with respect to current immunization practice and the possible benefit derived from herd immunity limits interpretation of these findings in a modern-day population where the etiologies of bacterial infection and the invasiveness of the related pathogens have shifted significantly. This uncertainty increases the importance of the clinical assessment and assurance of reliable follow-up.

The reliability of other means of temperature assessment when compared with rectal temperatures in young infants is discussed separately. (See 'Other methods of temperature measurement' above and "Fever in infants and children: Pathophysiology and management", section on 'Site and method of measurement'.)

Fever in a bundled infant — When ambient temperature and humidity permit normal heat loss, an elevated rectal temperature >38°C (100.4°F) should not be attributed to bundling in infants ≤90 days of age. In addition, a fever >38.5°C (101°F) should not be attributed to bundling regardless of the manner taken. When evaluating an infant who is afebrile by rectal temperature at presentation but has a fever at home, bundling is an important factor to consider if the fever is based upon temperature measurements other than a rectal temperature. (See 'Caregiver measurement' above.)

When ambient temperature and humidity pose a significant threat to heat loss (eg, ambient temperature >35°C [95°F] and/or relative humidity >75 percent), infants who are bundled have a limited ability to acclimate and can have elevated temperatures >38°C (100.4°F) that indicate heat illness. However, infection is still an important consideration. (See "Heat illness (other than heat stroke) in children", section on 'Pathophysiology' and "Heat stroke in children", section on 'Epidemiology'.)

Bundling of infants in clothing or blankets generally causes an elevation in skin temperature rather than rectal temperature when ambient conditions permit normal heat loss. As an example, in a randomized trial of 64 infants 11 to 95 days of age, rectal temperature after one hour was not significantly different between bundled and unbundled infants but mean skin temperature was 1.2°C higher in bundled infants [15]. Thus, bundled young infants outside of the newborn period may feel warm, but typically their rectal temperature will reflect their true core temperature. Again, this stresses the importance of using rectal measurements in young febrile infants rather than temporal artery, axillary, or other methods of temperature assessment that may be more impacted by alterations in skin temperature. (See 'Rectal temperature' above.)

SOCIETY GUIDELINE LINKS — 

Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Febrile young infants (younger than 90 days of age)".)

INFORMATION FOR PATIENTS — 

UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5^th to 6^th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10^th to 12^th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.

Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.)

●Basics topic (see "Patient education: Fever in babies younger than 3 months (The Basics)")

●Beyond the Basics topic (see "Patient education: Fever in children (Beyond the Basics)")

SUMMARY AND RECOMMENDATIONS

●Definition – Rectal temperatures are the gold standard for detecting fever in young infants. A rectal temperature ≥38ºC (100.4ºF) defines fever in neonates and infants 90 days of age or younger. (See 'Definition of fever' above and 'Rectal temperature' above.)

Axillary, temporal artery, and tympanic membrane temperatures appear less reliable for detecting fever in young infants. Axillary temperature may be used when rectal thermometry is contraindicated, such as in patients with neutropenia, bleeding diathesis (eg, thrombocytopenia), or necrotizing enterocolitis. (See 'Other methods of temperature measurement' above.)

●Approach – The approach to a report of fever in young infants depends on how the temperature was taken. However, any infant ≤90 days old in whom fever is suspected should undergo a careful history, physical examination, and appropriate ancillary studies according to age and clinical findings if fever is confirmed or strongly suspected by caregiver report.

•Rectal temperature – Young infants ≤90 days old who have a rectal temperature ≥38ºC (100.4ºF) should undergo a careful evaluation for the fever source and the likelihood of serious bacterial or viral infection according to age and clinical findings. (See "The febrile neonate (28 days of age or younger): Outpatient evaluation" and "The febrile infant (29 to 90 days of age): Outpatient evaluation".)

•Non-rectal routes – Temperatures obtained by non-rectal routes should be confirmed by rectal temperature measurement in young infants except when rectal thermometry is contraindicated such as in patients with neutropenia, bleeding diathesis (eg, thrombocytopenia), or necrotizing enterocolitis. When the measurements are close in time (ie, within 15 to 30 minutes), then we use the rectal temperature to determine if a fever is present. If the measurements are not close in time, then if either measurement indicates a fever, we proceed with an evaluation according to the patient age and clinical findings. (See 'Other methods of temperature measurement' above.)

For infants who have contraindications to rectal temperatures and have an axillary temperature near but not greater than 38°C (100.4°F), options include serial axillary temperature measurement and observation, or laboratory evaluation for infection. (See 'Other methods of temperature measurement' above.)

•Caregiver measurement – An infant with a reliable caregiver report of fever should undergo appropriate evaluation even if the infant is afebrile at presentation including (see 'Caregiver measurement' above):

-A history of objective fever by rectal temperature measurement.

-For elevated temperature within the past 24 hours that is based upon other non-rectal temperature measurements, our approach is to proceed with an evaluation according to the patient age and clinical findings regardless of the rectal temperature at clinical presentation or the timing between measurements by the caregiver and the medical provider. (See "The febrile infant (29 to 90 days of age): Outpatient evaluation", section on 'Evaluation' and "The febrile neonate (28 days of age or younger): Outpatient evaluation".)

•Tactile fever – An afebrile infant with a reported history of subjective (tactile) fever may avoid laboratory testing if all of the following conditions are met (see 'Caregiver measurement' above):

-Rectal temperature is normal.

-No antipyretic medication (eg, acetaminophen) has been given.

-History does not suggest an increased risk for bacterial infection (eg, no perinatal risk factors such as prematurity, prolonged rupture of membranes, recent antibiotics, or maternal colonization with Group B streptococcus; normal behavior and feeding; and no associated difficulty breathing, apneic spells, or color changes).

-Appearance and physical examination are normal.

-Reliable follow-up within 12 to 24 hours can be arranged.

-Caregivers can monitor the rectal temperature at home, understand indications to seek medical attention, and have transportation.

For patients in whom reliable follow-up is not assured and who have not received antipyretic therapy, observation for lack of fever by rectal measurement at one or two additional time points in the emergency department (eg, for one to two feeding cycles) remains an option.

●Bundling – An elevated rectal temperature should not be attributed to bundling in young infants ≤90 days of age when ambient conditions permit normal heat loss. In addition, a fever >38.5ºC (101ºF) should not be attributed to bundling regardless of the manner taken. (See 'Fever in a bundled infant' above.)