A doctor may consider a urine test, blood tests, a swab for viruses or strep throat, or an X-ray, depending on the child’s age, symptoms, examination, and how long the fever has lasted. Many older children who look well do not need tests. Babies under 3 months usually need prompt medical assessment and more testing because infections can be harder to spot at that age.
How does a doctor decide which tests a child needs?
Testing usually begins with questions about when the fever started, how the temperature was measured, and whether other symptoms have appeared. The doctor may ask about coughing, vomiting, diarrhea, pain when passing urine, recent vaccinations, medications, travel, and contact with someone who is ill. The physical examination may include checking the ears, throat, chest, abdomen, skin, hydration, and breathing.
According to the Merck Manual, the need for tests depends on factors such as the child's age, overall appearance, vaccination status, and whether the fever is recent or persistent. Many older children with short-term fever and signs of a common respiratory infection can be assessed without extensive testing. The aim is to choose tests that answer a specific clinical question rather than order every available test.
Which tests might a doctor consider for a child with fever?
1. Urine test and urine culture
A urine test is often considered when a child has a fever without an obvious cause. A urinary tract infection (UTI) can sometimes cause fever without the usual urinary symptoms, particularly in babies and young children who cannot explain what they feel. A urinalysis checks the urine for findings that may suggest infection, while a urine culture looks for bacteria and helps identify the organism responsible.
The American Academy of Pediatrics explains that urine testing can help evaluate a possible UTI and other concerns. Children who use the toilet may provide a sample in a clean container. For babies who wear diapers, a healthcare professional may recommend a collection method suited to the child's age and the test being performed.
2. Blood tests
Blood tests may be considered when the examination raises concern about a bacterial infection, inflammation, or another illness that needs further investigation. The particular tests depend on what the doctor is trying to find out.
- Complete blood count (CBC): Measures different blood cells, including white blood cells, which help the body respond to infections. The result is interpreted alongside the child's symptoms and examination.
- C-reactive protein (CRP) and procalcitonin: These are markers that may help assess inflammation and the possibility of a significant bacterial infection in selected children.
- Blood culture: A blood sample is sent to a laboratory to check whether bacteria or other specific microorganisms grow in it. This may be considered when a serious infection is suspected.
- Other blood tests: Depending on the clinical findings, a doctor may check electrolytes, kidney function, liver function, or other markers relevant to the suspected cause.
MedlinePlus describes blood tests, including a CBC, as possible investigations during a fever assessment. These tests do not establish a diagnosis on their own. For example, a white blood cell result can be affected by different illnesses, and a raised inflammatory marker does not identify the exact cause of a fever.
3. Swabs for respiratory viruses
If a child has a cough, runny nose, sore throat, or other respiratory symptoms, the doctor may consider testing for viruses such as influenza, respiratory syncytial virus (RSV), or the virus that causes COVID-19. The sample is commonly collected from the nose or another part of the respiratory tract. Depending on the test, the laboratory may look for viral proteins or genetic material.
The Centers for Disease Control and Prevention (CDC) explains that respiratory virus testing may help guide healthcare decisions. Some tests provide results quickly, while molecular tests such as polymerase chain reaction (PCR) tests detect genetic material and may be more sensitive than antigen tests. A negative result does not always exclude an infection, so the doctor interprets it in context.
4. Throat swab or rapid strep test
When a fever occurs with a sore throat or examination findings that raise concern about a group A streptococcal infection, a doctor may recommend a rapid strep test or a throat culture. A swab is taken from the back of the throat and tonsil area. A rapid test can provide an answer relatively quickly, while a throat culture requires time for organisms to grow.
Whether a throat test is useful depends on the child's age, symptoms, and examination. Fever alone does not mean that a child needs a strep test, and antibiotics are not appropriate for every sore throat. The healthcare professional uses the examination and test results to decide whether further care is needed.
5. Chest X-ray
A chest X-ray may be considered when a doctor suspects pneumonia or another problem affecting the lungs. Signs such as difficulty breathing, an abnormal breathing rate, or concerning findings during the chest examination may influence this decision. A chest X-ray produces images of the lungs and surrounding structures and can help identify certain changes associated with infection.
Most children with a short fever and no concerning respiratory findings do not automatically need a chest X-ray. The decision depends on the examination and the likelihood that imaging will provide useful information.
6. Stool tests
A stool sample may be tested when fever occurs with diarrhea and the doctor suspects a gastrointestinal infection that needs further investigation. Depending on the circumstances, testing may look for particular bacteria, viruses, or parasites. Stool testing is not routinely necessary for every child with diarrhea; the nature and duration of the illness, associated symptoms, exposure history, and the child's overall condition help determine whether it is useful.
7. Lumbar puncture and spinal fluid testing
A lumbar puncture, sometimes called a spinal tap, collects a small sample of cerebrospinal fluid, the fluid surrounding the brain and spinal cord. The sample can be tested for signs of infection, including meningitis. This procedure is not a routine test for every child with fever, but it may be considered when the child's age, clinical condition, or examination raises concern about an infection involving the nervous system.
The Merck Manual's guidance on fever in children includes spinal fluid testing among the investigations used for selected suspected conditions. The decision is made by the treating team after assessing the child and considering the urgency of the situation.
Why do babies under 3 months often need a different evaluation?
A fever in a baby younger than 3 months requires prompt medical assessment, even when the baby initially appears well. In this age group, a serious infection may be difficult to recognize from symptoms alone, so clinicians generally have a lower threshold for testing. Evaluation may include urine testing, blood tests, blood cultures, and markers of inflammation. Depending on the baby's exact age, appearance, test results, and clinical risk assessment, a lumbar puncture or additional investigations may also be recommended.
The American Academy of Pediatrics advises prompt medical attention for a baby younger than 3 months with a temperature of 100.4°F (38°C) or higher. Do not delay seeking care to see whether the fever goes away or to arrange home testing. The clinician will determine which investigations are appropriate and whether the baby needs observation or hospital care.
What happens if the fever keeps returning or does not go away?
If a fever persists, returns repeatedly, or remains unexplained after the initial examination, the doctor may reconsider the possible causes and decide whether additional tests are needed. The next steps depend on the length and pattern of the fever, the child's growth and general health, associated symptoms, travel or exposure history, and earlier test results.
Further investigations might include repeat blood tests, additional cultures, targeted infection testing, or imaging such as an ultrasound or other scan when the examination points toward a particular area of concern. Persistent fever does not automatically mean that an uncommon or serious disease is present. The purpose of further investigation is to answer specific questions raised by the child's clinical course.
Does every child with a fever need laboratory tests?
No. Some children can be assessed through their history and physical examination alone, especially when they are older, appear well, and have symptoms consistent with a common short-lived illness. Other children need testing because of their age, concerning examination findings, an unclear source of fever, or a medical condition that affects their risk of infection.
Tests also have limits. Results may take time, a test can be negative even when an infection is present, and an abnormal result may need additional investigation before its meaning becomes clear. The doctor balances the expected benefit of testing against factors such as discomfort, the chance of an unclear result, and whether the result would change care.
When should a child with fever receive urgent medical attention?
Seek urgent medical assessment if a baby younger than 3 months has a temperature of 38°C (100.4°F) or higher. For a child of any age, urgent assessment is also important when fever occurs with difficulty breathing, unusual drowsiness or difficulty waking, a stiff neck, a seizure, a rash that does not fade when pressed, or a marked change in responsiveness. Repeated vomiting, signs of dehydration, or a child who is becoming more unwell also warrant prompt medical advice.
In an emergency, do not wait for a routine appointment or try to complete tests at home. The healthcare team can assess the child's condition, start any necessary urgent care, and select investigations based on the findings.