An evaluation of a fever in an infant begins with a medical history and physical examination.

Options for measuring the temperature include:

• Rectal temperature:
• Very reliable
• Best option for infants
• Oral thermometer:
• Unreliable results in infants
• Under the arm temperatures:
• Unreliable results
• Thermometer ear probe:
• In some cases can provide unreliable results
• Thermometer skin patches:
• Unreliable results

Test may be required in the following infants:

• An infant less than 2 months old, who has a fever over 100.4 degrees F (38 C)
• An infant less than 2 years old, who has a fever over 101.9 degrees F (38.8 C)

Tests that may be used to evaluate an infant less than 3 months old, who has a fever over 100.4 degrees F (38 C) include:

• Complete blood count
• Kidney profile
• Blood cultures
• Spinal tap:
• Encephalitis or meningitis may cause the spinal fluid to become cloudy
• Chest x-ray
• Urinalysis
• Urine cultures
• Serologic tests for certain bacteria, fungi and viruses

Tests that may be used to evaluate an infant less than 2 years old, who has a fever over 101.9 degrees F (38.3 C) include:

• Complete blood count
• Blood cultures
• Chest x-ray
• Urinalysis
• Urine cultures
• Serologic tests for certain bacteria, fungi and viruses

Additional tests may be performed, based on the presence of additional symptoms.