People with cancer can get many different types of infections. These infections differ in their risk factors, the symptoms they cause, how they are treated, and the chance for curing the infection. When looking at the different types, it is important for the doctor to know:
- The part of the body affected
- The type of germ causing the infection
Parts of the body most likely to get infections
Your signs and symptoms (for example, where you have pain, redness, or swelling) help your doctor know what tests are needed to find the cause of the infection. The results of certain tests (such as x-rays, CT scans, or lab tests done on body fluids) help pinpoint where the infection is.
Common sites of infection in people with cancer include
- The skin and mucous membranes
- The digestive system (esophagus or swallowing tube, stomach, intestines, and rectum)
- The lungs and breathing passages (sinuses, throat, and lungs)
- The urinary system (bladder and kidneys)
- The nervous system (brain and spinal cord)
- The skin and tissue around a vascular access device (VAD). The VAD is a tube or catheter put in a vein that is used to draw blood and give IV medicines or fluids (like a PICC line or port-a-cath).
Finding the germs that cause infections in people with cancer
Infections are grouped by the type of germs that cause them. Bacteria, viruses, protozoa (some of which act as parasites), or fungi may cause infections in people with cancer.
Most of the infections in people with cancer are caused by germs that normally live on the skin, in the intestines, or in the environment. These germs usually do not cause infections in people with normal defenses and immune systems. But if the normal barriers and immune system are weak, the germs can begin to grow and cause damage. These infections are often called opportunistic infections, because the germs use the opportunity of a patient’s weakened defenses to cause illness.
Naming germs
Like all other living things, germs are given scientific names from the genus (first names) and species (last names). These names tell biologists and health care workers which things are related and may be like each other. For example, the biological names Felis domesticus (house cats) and Felis leo (lions) tell biologists the animals are related. (Note that scientific names are usually written in italics, except for viruses.)
In the same way, Staphylococcus epidermidis and Staphylococcus aureus are related bacteria. But, like house cats and lions, they differ in important details. Staphylococcus epidermidis lives on our skin and rarely causes infections in healthy people. But it can cause infections in people with cancer who have an IV (intravenous) or vascular access device (VAD) and a low white blood cell count. Staphylococcus aureus can cause very serious infections even in those with healthy immune systems. It is often resistant to certain antibiotics and can be very hard to treat.
Although most germs have first and last names, those that are very well known are often called by their first names only. Germs like this include Pneumococcus, Candida, and Aspergillus. And like some people, germs with long names are sometimes more commonly known by their initials or nicknames. These include E. coli (Escherichia coli), Staph epi (Staphylococcus epidermidis), and CMV (cytomegalovirus).
Lab tests to identify germs
Knowing the exact type of germ that is causing an infection helps doctors choose the best treatment. Different drugs are used to treat each of the main types of germs — bacteria, viruses, fungi, and parasites. And even among the main types of germs, different types are treated by different drugs. This means an antibiotic that can kill one type of bacteria might have no effect on another type of bacteria. Your health care team will do all they can to find out exactly what germ is causing your infection. Here are some of the tests they may use.
Gram stain: Samples from the suspected site of infection can be put on a microscope slide and treated with a series of stains (dyes). This test is called a Gram stain. It changes the colors of the germs and makes them easier to see. It takes only a few minutes to stain the sample and look at it. The colors and shapes of the stained germs often give doctors an idea of which germ is causing the infection. This can help narrow the choice of antibiotics until more precise test results come in. Gram stains are most useful for bacteria; fungi and parasites usually need different stains.
Tests for viruses: Viruses are too small to be seen with most microscopes. Instead, viruses are sometimes identified by lab tests that look for the viral proteins or viral genes. Others are detected by finding their antibodies.
Genetic tests: New medical lab tests can quickly identify some germs by testing their DNA, RNA, or other substances. These tests often help doctors choose the best treatment more quickly than older tests. This means they can help get the right treatment started before the infection causes serious harm.
Culture: Samples from the suspected site of infection are put in a nutrient broth or gel and kept warm until the germs grow. Bacteria and fungi most often need many days to grow in the lab before they can be seen. After enough germs have grown, they are tested so they can be identified. Viruses may also be identified by taking samples and growing them in the lab, although it can take a few weeks to get results.
Sensitivity tests: Once bacteria have been cultured, an extra test called a sensitivity test may be done. This is often needed because some germs resist certain antibiotics that kill others of that same type. This test also takes time, but it shows the best antibiotic to kill the germ causing a certain infection. Because infections in people with cancer may quickly get worse, treatment is usually started before culture and sensitivity results are back. These antibiotics may be changed after the lab tests have identified the exact germ and what will work best to treat it.
But how do doctors treat the infection while they wait for lab test results?
Doctors know which germs tend to infect which organs of people with cancer. So they can often make an educated guess at which germs are most likely causing a patient’s infection. Sometimes they are able to use tests that can be done quickly, like the Gram stain, to narrow down the type of germ. Educated guesses are very important because it can take many days to get the results of the tests that show the exact type of germ causing an infection and what drug will best stop or kill it.
Test samples or specimens
All of the tests listed above are done on samples or specimens from the patient. Common types of samples include:
- Blood
- Urine
- Spinal fluid
- Wound drainage or pus
- Phlegm (sputum)
If you have symptoms that point to a certain organ, samples are taken to check for germs in that area. For example, sputum samples may be taken if you have a cough or are short of breath. Urine samples may be taken if you have blood in your urine or feel pain while urinating. Sometimes, if a person has a very low white blood cell count and a fever, samples will be taken before these types of symptoms start.
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