Initial evaluation of prolonged fever

1. Introduction

Fever is one of the most common symptoms in pediatrics and is estimated to account for approximately one-third of all pediatric visits in primary healthcare. In children, elevated body temperature is defined as follows: rectal temperature above 37.9°C, axillary temperature above 37°C, oral temperature above 37.5°C, and ear temperature above 37.9°C for children under 11 years of age and above 37.5°C for children older than 11 years. Within pediatric practice, fever can be categorized into several types.

When fever lasts up to seven days, it is considered a short-term febrile condition. This group includes children who, in addition to an elevated temperature, may exhibit other symptoms or signs indicating the source of infection, referred to as fever with localized symptoms. Conversely, if a child has a fever lasting up to seven days without pronounced additional symptoms, it is classified as a short-term, unclear febrile condition, also known as fever without localizing signs (FWLS), fever without a source (FWS), or fever without a focus (FWF). In most of these cases, the cause is a “self-limiting infection” that does not require specific therapy unless a bacterial infection is confirmed, in which case antimicrobial therapy is necessary.

Particular caution is required for neonates and infants younger than three months, as they are at a significantly higher risk of serious bacterial infections, such as sepsis, meningitis, bacteremia, urinary tract infections, or osteomyelitis. Due to the complexity of clinical assessment, fever in this age group often necessitates hospitalization for extensive diagnostics, monitoring, and treatment.

Children with a fever lasting eight or more days, for whom history, clinical examination, and basic laboratory findings do not identify a cause, are considered to have a fever of unknown origin or an unexplained febrile condition. This classification has been adapted for pediatric practice because the original definition of this condition, established in 1961, was primarily intended for adults. According to this definition—based on a prospective study conducted by Petersdorf and Beeson at Yale University—fever of unknown origin was defined as a body temperature above 38.3°C lasting more than three weeks without an identifiable cause after one week of intensive hospital investigations. Over time, various adaptations of this definition have been proposed, including distinctions based on outpatient follow-up (where fever must persist for at least three weeks) versus inpatient care (where this period is reduced to seven days) or the introduction of a standardized two-week duration regardless of the method of patient follow-up.

2. Etiology

Certain inflammatory mediators, such as interleukin-1β (IL-1β), interleukin-6 (IL-6), and tumor necrosis factor-alpha (TNF-α), play a key role in raising the thermostatic set point in the anterior hypothalamus. These so-called endogenous pyrogens are produced as a result of the inflammatory process, which can be triggered by infections, systemic non-infectious inflammatory diseases, or malignant tumors. The actions of IL-1 and IL-6 involve the activation of metabolic pathways, including cyclooxygenase-2 (Cox-2) and STAT-3, leading to the synthesis of prostaglandin E2 (PGE2). PGE2, in turn, raises the hypothalamic thermostat settings, resulting in fever.

The causes of fever of unknown origin (FUO) can be classified into four main categories: infections, non-infectious inflammatory conditions, neoplasms, and other causes. Despite thorough investigations, a variable percentage of patients remain undiagnosed. Over the years, the general categories of FUO have remained the same, though their proportions have changed. Early American studies on pediatric patients with FUO found that a cause was identified in about 80% of cases: infections accounted for 40%, inflammatory conditions of non-infectious etiology for 15%, and oncological causes for 10%. In the past decade, advancements in diagnostic methods have reduced the proportion of undiagnosed cases, with a cause now being identified in approximately 85% of cases. The largest systematic review to date in the pediatric population found that infections accounted for 51% of cases, with bacterial causes comprising 56–71% and viral causes 3–15% (with viral infections being more common in developed countries). The second most frequent category (23%) was undetermined etiology, which was more common in developed countries than in developing countries (31% vs. 18%). Other identified causes included various diseases (11%), autoimmune or autoinflammatory conditions (9%), and malignant diseases (6%).

The distribution of FUO causes differs between developing and developed countries. However, even in recent studies from developed nations, infections remain the most common cause, ranging from 37.2% to 50.9%, followed by inflammatory diseases (4.9% to 27.5%) and neoplastic diseases (4.5% to 17%). In developing countries, brucellosis is a leading cause of FUO, followed by typhus, rickettsiosis, and tuberculosis, whereas in developed countries, bacterial infections such as osteomyelitis, bartonellosis, and viral infections—particularly Epstein-Barr virus (EBV)—are more prevalent. Additionally, FUO causes vary by age group. Infections are the most common cause in the first year of life, decreasing in proportion as children grow older, though they remain the leading cause overall. A recent Chinese study found that the proportion of infections decreased from 73% in infants to 44% in older children, with bacterial infections being predominant in children under one year of age and viral infections in those aged one to six years. Oncological and autoimmune diseases become more common in later childhood. Furthermore, the longer a fever persists, the less likely it is to be of infectious origin—especially if it lasts longer than four weeks.

A 1998 study by Jacobs and colleagues identified EBV infection as the most common infectious cause of FUO, followed by osteomyelitis, Bartonella infection, and urinary tract infections. Infectious diseases that manifest as prolonged fever can be categorized into systemic and localized infections. Systemic infections are most often caused by viruses from the herpesvirus group, including EBV and cytomegalovirus (CMV). Other systemic viral infections include HIV and viral hepatitis. Among bacterial pathogens, tuberculosis, salmonellosis, rickettsiosis, brucellosis, tularemia, and spirochetal infections (e.g., Lyme disease and leptospirosis) can present with prolonged fever. However, unrecognized or inadequately treated localized infections—such as otitis media, lower respiratory tract infections, urinary tract infections, osteomyelitis, or hidden abscesses (e.g., in the abdomen)—can also present as FUO.

Among inflammatory rheumatic diseases, the most common febrile condition is juvenile idiopathic arthritis (JIA), particularly its systemic form (sJIA). Other significant autoimmune diseases include childhood-onset systemic lupus erythematosus (cSLE) and various types of vasculitis, particularly polyarteritis nodosa (PAN) and Kawasaki disease.

A distinct category of febrile conditions includes periodic or recurrent fever syndromes, classified as autoinflammatory diseases. These disorders are characterized by excessive inflammation mediated by the innate immune system, often with a genetic predisposition. To meet the diagnostic criteria, at least three episodes of unexplained fever must occur at intervals greater than seven days over a six-month period. Fever episodes are often high (≥39°C), and their intervals can be regular or irregular. Between episodes, the child is typically well and exhibits normal growth. During febrile episodes, laboratory findings show leukocytosis and elevated acute-phase inflammatory markers, which normalize when the fever subsides. These syndromes typically begin within the first 10 years of life, with a positive family history in many cases. Ethnic background can also play a role in predisposition. Based on episode duration, periodic fever syndromes can be categorized as follows:

• Short episodes (24–48 hours): Familial Mediterranean fever (FMF)
• Medium-length episodes (4–7 days): Mevalonate kinase deficiency (MKD)
• Prolonged episodes: TNF receptor-associated periodic syndrome (TRAPS)

The most common periodic fever syndrome is PFAPA syndrome (Periodic Fever, Aphthous Stomatitis, Pharyngitis, and Adenitis). It usually begins before the age of five, with fever episodes lasting three to five days and occurring approximately every 28 days. Episodes may be accompanied by aphthous stomatitis, tonsillopharyngitis, and cervical lymphadenitis.

Among malignant diseases that can present as FUO, lymphomas and acute leukemias are the most notable. Hodgkin lymphoma typically has a slower onset than other pediatric malignancies and is characterized by lymphadenopathy in the neck and mediastinum, fatigue, weight loss, and subfebrile or high fever (>38°C), which rarely presents with chills and usually resolves spontaneously within a few hours. Acute leukemias, the most common pediatric malignancies, often manifest with febrile episodes, usually due to infection. Other malignancies that can present with FUO include neuroblastoma, primitive neuroectodermal tumors, Ewing’s sarcoma, and central nervous system tumors.

3. CLINICAL APPROACH TO THE CHILD WITH FEVER OF UNKNOWN ORIGIN

The evaluation of fever of unknown origin (FUO) should be systematic and guided by a thorough medical history and clinical examination (Tables 1 and 2). A detailed history, along with an objective physical examination and interpretation of initial laboratory findings, is the primary focus.

Table 1. Peculiarities of the medical history in children with fever of unknown cause

Table 2. Physical examination characteristics in children with fever of unknown cause

3.1. TARGETED HISTORY

The first step in obtaining a proper history is to define the fever in terms of its duration, peak temperature, and pattern. It is essential to confirm the presence of fever, as parental perception often differs from the medical definition. Parents may report a tactile or subjective fever without actually measuring the temperature. Therefore, it is important to determine what they consider a fever and whether this aligns with the standard medical definition of ≥38.0°C. Parents should be asked whether the temperature was measured with a thermometer and what factors influence its reduction (e.g., paracetamol is often ineffective in reducing fever associated with Kawasaki disease).

A detailed description of fever patterns can help narrow the differential diagnosis:

• Intermittent fever: Seen in conditions such as tuberculosis and systemic juvenile idiopathic arthritis (sJIA)
• Recurrent fever: Characteristic of periodic fever syndromes and spirochetal infections
• Remittent fever: Associated with conditions like endocarditis
• Continuous fever: Suggestive of pyogenic abscesses

FUO should be distinguished from pseudofever of unknown origin, which refers to consecutive episodes of benign, self-limited infections that parents perceive as a single prolonged febrile illness. This differentiation is crucial to avoid unnecessary and costly investigations. Pseudofever of unknown origin often begins with a well-defined viral infection that resolves but is followed by additional febrile viral illnesses, which may be less well-defined. A careful history focusing on identifying afebrile intervals between fever episodes is essential, though distinguishing it from true FUO can be challenging.

The medical history should also include information on ethnicity and geographical background. A travel history should be obtained, not only for recent trips but also for past travels, as some infections can have prolonged incubation periods. Additionally, exposure to domestic or wild animals and insect bites should be investigated, as zoonotic diseases have been increasingly reported even in developed countries.

Further history-taking should assess potential exposure risks, including:

• Contact with sick individuals or high-risk environments (e.g., recent travel abroad, visits to prisons, or homelessness)
• Dietary habits (e.g., consumption of unpasteurized dairy products, well water, or undercooked meat)
• Possible parasitic infections, such as toxocariasis, which may be linked to exposure to contaminated soil or pets
• Adolescent risk factors, including sexual activity, body piercings, tattoos, and drug use
• Vaccination history, including the most recent immunizations

It is also important to determine whether the child has underlying comorbidities, recurrent infections (which may suggest immunodeficiency), implanted medical devices (e.g., central venous catheters, percutaneous endoscopic gastrostomy), or recent surgical procedures.

Finally, the presence of systemic symptoms should be explored, including anorexia, weight loss, pruritus, joint pain, abdominal pain, and changes in stool (e.g., diarrhea or melena), as these may provide clues to the underlying cause of the fever.

3.2. PHYSICAL EXAMINATION

A thorough physical examination, including assessment of vital signs and body weight, is essential. Certain clinical signs can be highly suggestive of specific diseases, making repeated physical examinations critical. In some cases, inpatient observation in a controlled setting may be beneficial, as up to 25% of significant physical findings may be absent during the initial evaluation.

Conversely, a completely normal physical examination at the time of initial assessment in a child with unexplained fever is highly suggestive of a benign cause.

3.3. DIAGNOSTIC PROCEDURE

Although children with fever of unknown origin (FUO) often undergo extensive diagnostic workups, research has shown that the most important step in establishing an accurate diagnosis is obtaining a detailed history and performing a meticulous physical examination. According to some studies, omissions in history-taking and examination have led to errors in the final diagnosis in 10% to 15% of cases.

Furthermore, while rare and unusual diseases are often considered when evaluating a child with an unclear febrile condition, research indicates that common diseases presenting in atypical ways are far more frequent. As a result, complex and costly diagnostic tests are generally needed less often in children than in adults. To ensure rational and efficient management, diagnostic testing should be conducted gradually and systematically.

The decision to manage a child with an unexplained febrile illness as an outpatient or inpatient should be based on the child’s general condition, the presence or absence of abnormal physical findings, and history of comorbidities or risk factors.

• Children without danger signs and with a normal physical examination (i.e., stable vital signs) can undergo an outpatient workup that includes first-line laboratory tests and basic imaging.
• Children with abnormal clinical findings or concerning historical factors should undergo evaluation in the hospital.

3.3.1. First-Line Investigations

The initial workup for children with FUO should include the following laboratory tests:

• Complete blood count (CBC) with peripheral blood smear
• Erythrocyte sedimentation rate (ESR)
• C-reactive protein (CRP)
• Liver function tests (hepatogram), including lactate dehydrogenase (LDH)
• Electrolytes
• Serum protein electrophoresis
• Blood cultures
• Urinalysis and urine culture

Additional tests may be required, depending on clinical suspicion, and may include:

• Procalcitonin (PCT)
• Urate and creatine kinase
• Ferritin levels
• Immunoglobulin (IgA, IgM, IgG) levels
• Stool test for occult blood
• Cytological, bacteriological, and parasitological analysis of stool
• Throat swab culture
• Tuberculin skin test (TST) or interferon-gamma release assay (e.g., Quantiferon test)

Based on history and clinical findings, synovial fluid analysis or lumbar puncture may also be warranted. Some experts recommend including serologic tests for CMV, EBV, Bartonella, and HIV in the first-line workup.

First-line imaging should include:

• Chest X-ray, even in the absence of respiratory symptoms
• Abdominal ultrasound
• Cardiac ultrasound (echocardiography)
• Electrocardiogram (ECG)

Children with musculoskeletal symptoms should first undergo bone X-rays, followed by musculoskeletal ultrasound if indicated.

Laboratory Findings and Their Clinical Significance

1. Complete Blood Count (CBC) Abnormalities

Common abnormalities in the CBC and their potential causes include:

• Anemia: Rheumatic diseases (e.g., JIA, cSLE, Kawasaki disease), infections (e.g., malaria, tuberculosis, endocarditis), inflammatory bowel disease, malignancies
• Cytopenia in multiple cell lines / immature cell forms: Leishmaniasis, leukemia, hemophagocytic lymphohistiocytosis (HLH), cSLE
• Atypical lymphocytes: EBV, CMV, Kikuchi-Fujimoto disease, lymphoproliferative disorders
• Lymphocytosis: Cat-scratch disease, EBV infection, toxoplasmosis
• Eosinophilia: Parasitic or fungal infections, allergies, neoplasms, immunodeficiencies, drug-related fever
• Thrombocytosis: Juvenile idiopathic arthritis (JIA), Kawasaki disease
• Thrombocytopenia: Viral infections, leptospirosis, tularemia, malaria, autoimmune diseases (e.g., cSLE)

2. Inflammatory Markers (WBC, ESR, CRP, PCT)

Inflammatory markers are essential tools in the evaluation of FUO, though they lack disease specificity:

• Elevated leukocytes, ESR, CRP, or PCT suggest inflammatory or infectious processes.
• CRP trends over time are more useful than a single value for diagnosis, monitoring response to treatment, and guiding therapy.
• ESR is influenced by several factors and may be falsely elevated in anemia, hypergammaglobulinemia, and hyperfibrinogenemia or falsely decreased in intravascular coagulopathy and HLH.
• CRP and PCT are more sensitive than ESR for detecting early inflammation, but PCT is most useful for severe bacterial infections and less reliable in advanced fever.
• Serum ferritin may serve as a useful biomarker to differentiate infectious vs. noninfectious causes of FUO. Ferritin levels ≥5 times the upper limit of normal have been proposed as a screening test for noninfectious inflammatory causes such as systemic JIA (sJIA).

3.3.2. Second-Level Investigations

If the history and/or physical examination provide certain clues or reveal danger signs, second-level investigations should be conducted, selected based on the suspected diagnosis. Hospitalization is not always required for these investigations. However, admission to the hospital should be considered in cases of:

• General deterioration or clinical instability
• Rapid progression of symptoms
• Clinical suspicion of Münchhausen syndrome by proxy
• Need for close monitoring
• Requirement for more complex investigations or procedures

During this second phase, if available, consultation with pediatric infectious disease specialists, rheumatologists, and/or hemato-oncologists is advisable, depending on the suspected condition.

Microbiological Workup

The microbiological workup, guided by clinical suspicion, includes specific viral and bacterial serology and PCR-based pathogen detection. This may include, in addition to the first-line serologies for CMV, EBV, Bartonella, and HIV, additional tests for:

• Viral hepatitis
• Toxoplasmosis
• Borreliosis
• Syphilis
• Tularemia
• Toxocariasis
• Brucellosis

Additionally, some authors include the Quantiferon test in first-line investigations. Malaria testing should be performed in any febrile child from endemic areas.

Immunological and Autoimmune Investigations

If immune-mediated diseases are suspected, immunological tests should be performed, including:

• ANA (antinuclear antibodies)
• ENA (extractable nuclear antigen) profile
• Anti-dsDNA antibodies
• ANCA (anti-neutrophil cytoplasmic antibodies)
• Rheumatoid factor (RF)
• Complement levels (C3, C4, CH50)
• Immunophenotyping of peripheral blood lymphocytes

For suspected rheumatic fever, a prior streptococcal infection should be ruled out by checking antistreptolysin titers in at least two evaluations.

For suspected inflammatory bowel disease (IBD), testing should include ANCA and fecal calprotectin.

Oncological and Metabolic Investigations

For suspected neuroblastoma (e.g., systemic hypertension, spinal/chest pain, opsoclonus-myoclonus syndrome), testing should include urinary vanillylmandelic acid (VMA).

For suspected pheochromocytoma, urinary metanephrines should be measured.

Invasive Diagnostic Procedures

If first-line investigations fail to establish a diagnosis or findings suggest a serious condition, invasive methods should be considered:

• Lymph node biopsy → in cases of persistent lymphadenopathy
• Liver biopsy → if ultrasound abnormalities or abnormal liver function tests are present
• Bone marrow aspiration/biopsy → in cases of:
  • Peripheral blood smear abnormalities
  • Poor general condition or progressive clinical deterioration
  • Fever associated with lymphadenopathy, hepatosplenomegaly, weight loss, or night sweats (to exclude lymphoproliferative disorders, HLH, or visceral leishmaniasis)

Second-Level Imaging

Advanced imaging, guided by clinical suspicion, may include:

• Computed tomography (CT)
• Magnetic resonance imaging (MRI)
• Scintigraphy
• Endoscopy
• Tissue biopsies

Key Imaging Considerations:

• Head CT is the gold standard for suspected sinusitis or mastoiditis, as occult sinusitis is a common cause of FUO in children.
• In cases of suspected intra-abdominal abscess, contrast-enhanced CT may be more sensitive than ultrasound.
• If musculoskeletal involvement is suspected and X-ray/ultrasound findings are inconclusive, MRI of the affected area should be performed.

Whole-Body MRI:

Several studies suggest that whole-body MRI may be a useful screening tool for children with unexplained fever, particularly when conventional imaging fails to provide a diagnosis. However, as whole-body MRI is non-specific, a positive finding requires further confirmation (e.g., biopsy, targeted MRI, or bone marrow aspiration).

Scintigraphy and PET/CT:

Scintigraphy is particularly recommended for suspected malignancy, especially if initial investigations are inconclusive. Among nuclear medicine procedures, fluorodeoxyglucose positron emission tomography (18F-FDG PET/CT) is the first-choice method, as it combines anatomical and functional imaging.

However, limitations of 18F-FDG PET/CT include:

• False-positive and false-negative results
• Radiation exposure, necessitating careful risk-benefit assessment
• Standardized use only for conditions with significant immune activation, such as HLH

Genetic and Molecular Investigations

Genetic testing plays a crucial role in diagnosing monogenic autoinflammatory diseases. Next-generation sequencing (NGS) allows for the analysis of:

• A single gene, if the clinical phenotype is well-defined
• Gene panels, covering multiple candidate genes
• Whole-exome or whole-genome sequencing, if broader analysis is required

Genetic variants identified through sequencing are classified as:

• Pathogenic (disease-causing)
• Likely pathogenic
• Variants of uncertain clinical significance
• Likely benign
• Benign

Only pathogenic and likely pathogenic variants have direct diagnostic and therapeutic relevance.

Emerging Biomarkers

Due to the diagnostic challenge of distinguishing systemic juvenile idiopathic arthritis (sJIA) from infections, new biomarkers are needed.

A promising candidate is serum/plasma calprotectin, which has shown potential as a diagnostic and disease activity marker for sJIA. Research suggests that:

• Calprotectin levels can aid in sJIA diagnosis in febrile children.
• It may help assess disease activity, predict structural progression, and monitor treatment response.

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