Tests to Diagnose Infection

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How are infections diagnosed?

When doctors suspect an infection in a childhood cancer patient, they may need to find out what germ is causing the infection. Knowing the source of infection can help doctors determine an appropriate treatment.

An infection can be life-threatening for patients. Certain cancers and treatments can severely damage the immune system, leaving patients vulnerable to serious infections.

Figuring out the source of infection may require certain tests and procedures. The specific type depends on the patient’s signs and symptoms and what type of infection doctors suspect.

Because of the critical nature of infections, doctors may immediately begin a broad-spectrum antimicrobial treatment plan that can kill or stop the growth of a wide range of germs – even before the care team begins tests and procedures to pinpoint the cause of the infection.

Medical history and physical exam

Health care providers generally start by taking a detailed medical history and performing a physical examination. They are looking for signs and symptoms that indicate the site of infection. These may include, but are not limited to:

Skin lesions or ulcers
Pain in the pelvic area or abdomen
Facial or sinus pain
Eye redness or swelling
Respiratory signs and symptoms

After the examination, providers may order certain tests to gather more information.

Tests and procedures

Blood culture

Blood culture is the most common test to diagnose infection. It involves drawing blood from the patient and sending the blood sample to the laboratory.

Additional tests may be run depending on the patient’s signs and symptoms.

Urinary infection tests

If a urinary infection is suspected, the care team will collect a urine specimen from the patient and perform urine tests such as urine culture and/or urinalysis.

Respiratory infection tests

If the patient has respiratory symptoms such as coughing or a runny nose, doctors may perform a nasopharyngeal (nay zo fa RIN jee ul) wash or swab. This is done to obtain a sample of cells and secretions from the space behind the throat connected to the nasal passage. This space is called the nasopharynx.

Nasopharyngeal wash - To collect a sample of nasopharyngeal secretions and cells, a member of the care team will squirt saline (sterile salt water) into the back of the patient’s throat. This wash is used to help loosen the cells so it is easier to collect the sample. A soft, flexible catheter (tube) is attached to a syringe filled with saline. When it is time to collect a sample, a care team member may ask the patient to lie on his or her back with neck extended. This position allows the saline to pool at the back of the nasopharynx. Just before the procedure begins, the patient may be asked to hold his or her breath. A care team member may place the tube in the nose and gently insert it into the nasopharynx. A small amount of saline is flushed into the nasopharynx through the tube. Then, the staff member will pull back on the syringe to get a sample of cells.

Nasopharyngeal swab - A sterile cotton-tipped swab is gently passed through a nostril, along the floor of the nose, and into the nasopharynx. The swab is quickly rotated and removed.

In some cases, doctors may also order additional tests and procedures such as a chest X-ray, CT scan or bronchoscopy to check for sources of infection.

Skin infection tests

If doctors see signs of skin infection, they may perform a skin biopsy of the abnormal skin area to look for causes infections.

A skin biopsy involves removing skin cells from the suspected area of infection so the cells can be examined in the laboratory. Before the procedure is performed, a care team member will clean the area and may also apply a topical anesthetic cream to numb the skin. Sometimes a patient may receive an injection of lidocaine to numb the area. In some cases, the patient may also receive sedation medicine. After the skin cells are removed, there will be a small wound. It will be cared for and dressed to protect it. The care team will explain wound care and address any pain issues.

Wound infection tests

If a wound shows signs of infection, doctors will collect liquid from the wound and send it to the lab for testing.

To perform a wound culture, a member of the care team will clean the wound, collect fluid from the wound using a swab, place it in a culture preparation, and send to the lab.

Gastrointestinal infection tests

If a patient has signs and symptoms such as diarrhea, a stool specimen may be collected for culture or other types of tests.

A fresh stool sample must be collected in a clean container that has not been contaminated with other substances such as urine, water, or by touching the toilet or diaper. Once the sample has been collected, it will be taken to a laboratory. A stool sample may also be collected with a swab of the rectum.

Doctors may order a CT of abdomen and pelvis to look for signs of infection.

Central nervous system (CNS) infection tests

If doctors suspect a CNS infection, they may order a lumbar puncture test or imaging studies such as brain MRI.

At times, all test results may come back as negative. That means the type of the germ causing the infection could not be identified. If an infection is still suspected, doctors may, upon review, continue to use an antimicrobial treatment plan that is effective against most possible causes of the infection.

Susceptibility tests

If the germ causing the infection is detected growing on cultures, a susceptibility test may be performed. These tests determine if the germ will be susceptible or resistant to different antibiotics. The care team will use the results of these tests to determine the most appropriate treatment.

Getting results

It takes at least 24 hours and perhaps 2-3 days (or more) to get test results, depending on the type of test.

The care team will share the results with the family and discuss treatment options.