Health Library
Fever of Uncertain Source (Newborn)

What is Fever of Uncertain Source (In Baby 0 to 60 Days Old)?

Your baby has a fever above 100.4°F (38°C), and the reason for the fever is not obvious. Many things can cause a fever. The most common causes are bacterial or viral infections.

What can I expect when I take my baby to the doctor?

A caregiver will ask you questions about your baby’s illness and examine your infant. If the caregiver is concerned that your infant may have a serious infection, they may test your infant's urine, blood and/or spinal fluid to find the cause of the fever.

Testing Your Infant

Collecting Urine

The area where your infant urinates is cleaned. A small, flexible tube, or catheter, is placed into your infant’s bladder to get urine. We will take the catheter out as soon as we collect the urine sample. This sample will help the doctor check for any infection in your baby's urine.

Collecting Blood

We will place a thin plastic tube, called an IV, into a vein to collect some blood. We may leave this IV in the vein so we can give your baby fluids and medicines. This sample will help the doctor check for any infection in your baby's blood.

Collecting Spinal Fluid

A spinal tap is also called a lumbar puncture. It is done to remove a small amount of spinal fluid. Someone will hold your baby still during the procedure to keep them safe. The caregiver will clean your baby’s back and then place a needle between the bones in the lower part of the spine. The needle does not enter the spinal cord. The needle goes into the lower back below the level of the spinal cord. A small amount of fluid drips into tubes. We will remove the needle as soon as the fluid is collected. A small bandage will be placed over the area.

Test Results

We test all the fluid we collected to see if bacteria and / or viruses are present. Early test results will be available within two hours. Other tests, called cultures, check to see if bacteria grow from the blood, urine or spinal fluid. It takes 36 hours to find out if there are any bacteria growing.


There are many types of bacteria that can cause infections. These bacteria can be found in the urine, blood or spinal fluid. Your baby may receive antibiotics through the IV while you are waiting for the test results.


Like bacteria, there are many different viruses that can cause a fever. Most viruses cannot be treated with medications. Antibiotics work only on bacteria.

Information for the Doctor

It can help your child's caregiver to know:

  • How awake or sleepy your baby is
  • If your baby has a fever
  • If your baby is fussy or irritable
  • How well your baby is eating
  • If your baby is vomiting or having diarrhea
  • How many wet diapers your baby has had in the past 24 hours
  • If your baby has a rash or bumps on the skin
  • If your baby is having any breathing problems
  • If anyone in your household has recently been sick, has had a rash or has had cold sores

Last Updated 11/2021

Reviewed By Julie Snider, RN

Visiting Cincinnati Childrens.

Cincinnati Children’s has primary care services at locations throughout Greater Cincinnati.