Types of Infectious Disease Tests: An Overview

What are infectious disease tests?

There are many ways to diagnose infectious diseases. A healthcare provider will choose diagnostic tests depending on the types of infection and symptoms that patients experience. Tests include examinations, laboratory diagnostic tests, and imaging (scans). 

These tests are usually performed on blood, urine, stool, nasal or throat swabs, and tissue biopsies. Certain diseases or medical conditions may require a spinal tap (lumbar puncture) to collect cerebrospinal fluid from the spine for evaluation. Let’s have a look at various types of infectious disease tests.

Physical or visual examination

Photo by Bermix Studio on Unsplash

A physical examination is a vital first step in diagnosing infectious diseases. During this step, healthcare professionals can record the presence (and absence) of a patient’s symptoms, which helps them make a diagnosis. Signs of rashes or blisters, respiratory distress, a fever, and other symptoms can be noted. 

Patients can also use this time to tell medical professionals how they feel, inform them about recent travel, when their symptoms first appeared, and if they progressed over time. Healthcare providers may also palpate infected areas or listen to the lungs when they suspect respiratory conditions like pneumonia.

Blood tests

Photo by National Cancer Institute on Unsplash

Blood tests are widely-used methods to diagnose and confirm infections. 

In general, most laboratory tests (or assays) try to identify:  

• Antigen: An antigen is anything that triggers our immune response. For infectious diseases, antigen tests look for the disease-causing agent (pathogen)
• Antibodies: Specific antibodies generated by our bodies in response to the infection. Tests to detect antibodies in blood are also known as antibody serology tests.

The most commonly-used blood tests are: 

CBC (Complete Blood Count)

A CBC test measures blood cells in your body. A report usually provides a count of red blood cells (RBCs), white blood cells (WBCs), and platelets. The test is a fast and cost-effective screening test. 

When the blood count in a CBC is higher or lower than the norm, it can indicate infection and even help identify specific diseases—for example, viral infections like dengue lower platelet counts. Platelets play an essential role in blood clotting; a low platelet count increases the risk of bleeding. In severe cases, a low platelet count can lead to dengue hemorrhagic fever (DHF), which can be fatal without timely medical intervention. Monitoring platelet counts at regular intervals is critical to managing treatment. 

The CBC is often the first step in a series of diagnostic tests to confirm an infection. This is because white blood cell counts can be high due to many different bacterial, viral, or fungal infections. However, even auto-immune disorders like rheumatoid arthritis, certain cancers, and even inflammation can raise WBC levels. 

Blood culture tests

A blood culture test checks for bacterial and fungal infections. While a CBC offers information on our blood cell count, a blood culture test can identify the presence of specific bacteria or fungi in the blood. A healthcare provider may request a blood culture test if they suspect an infection in the blood. 

To run the test, a small blood sample is mixed with specific nutrients that help microbes grow to run the test. This mix is kept aside for a few days until the quantity of bacteria or fungi in the blood is large enough to identify the pathogen. Blood culture tests are important because they help medical professionals decide on the antibiotics needed for treatment. 

Blood culture tests are the gold standard in diagnosing bacterial and fungal infections in the blood. They are widely used, and it only takes a few days to receive test results. 

Polymerase chain reaction (PCR) or NAAT (Nucleic acid amplification test) 

PCR is a molecular test that detects DNA or RNA from biological samples. The PCR amplifies genetic material in blood, saliva, nasal swabs, or tissue samples. Medical professionals use the PCR to diagnose viral, bacterial, and parasitic infections.  

The PCR test is a game changer in infectious disease testing. In 1993, the inventor Kary B. Mullis even received a Nobel Prize in Chemistry for his invention. The test can generate millions of copies of DNA or RNA from even very small samples. It is highly sensitive and offers fast and accurate results, so it is a standard test to identify the presence of specific pathogens.

Urinalysis (urine tests)

Photo by Testalize.me on Unsplash

Urine tests help diagnose infections in the urinary tract using urine samples. 

Visual and microscopic examination 

Urine is typically clear, so examining urine samples visually and through a microscope can tell a lot about whether there may be an infection. If urine is cloudy, foamy, or has an odor or blood in it, chances are there may be an infection. 

When needed, urine may be evaluated under a microscope in the lab for signs of bacteria, yeast, or other infections. 

Dipstick test 

A dipstick is a thin strip with chemicals on it that is inserted into a urine sample. If the strip changes colors or indicates higher concentrations of specific elements in the urine, there may be an infection. For example, a higher concentration of nitrites may indicate a urinary tract infection. 

Like the visual exam, the dipstick test is a first-line screening test to learn if there may be an infection. In most cases, more diagnostic tests are needed to identify the disease-causing agent or related antibodies.  

Urine culture test

In a urine culture test, a small urine sample is grown in a laboratory in a controlled environment. This test can help diagnose bacterial infections in the urinary tract, and results are often delivered in a few days. Once the infection-causing bacteria are identified, healthcare professionals can select the right antibiotics to target the specific pathogen. 

Urine antigen test (UAT)

A urine antigen test can detect antigens produced in the body in response to a bacterial infection. For example, UATs can detect Legionella pneumophila, the most common cause of Legionellosis (Legionnaire’s disease), a severe respiratory infection. With early detection, medical professionals can treat infected individuals quickly and prevent the disease from progressing. 

Test results are usually available within a few hours, so the UAT is often used when a patient is hospitalized with a severe respiratory infection and needs a rapid diagnosis. 

PCR (Polymerase Chain Reaction)

A PCR test amplifies the DNA or RNA in small blood, urine, saliva, or other tissue swabs. 

The urine PCR is useful when a pathogen:

• Is in low concentrations in the urine sample, 
• Is difficult to grow in the lab (for urine culture test), or 
• The sample quantity is not enough for testing.

Stool tests

Photo by Diana Polekhina on Unsplash

Stool or fecal tests use stool (poop) samples to detect infections, inflammation, and bowel cancer. There are different types of stool tests based on the type of medical condition being diagnosed. 

Stool microscopic examination

Stool microscopic exams are often conducted when trying to diagnose the source of gastroenteritis (inflammation of the stomach and intestines). Once the disease-causing bacteria or other agent is identified, the infected person can be treated with the appropriate medicine. 

Stool culture test (Fecal culture test)

A fecal culture test is used to detect bacteria in stool samples. The bacteria in the stool is grown under controlled conditions in a lab and analyzed. Once identified, a clinician can treat their patient using the right antibiotics. 

Fecal immunochemical test (FIT)

A fecal immunochemical test detects hidden blood in stool samples, a potential sign of colorectal cancer. However, FIT tests can also detect gastroenteritis-causing bacteria like giardia. The test uses antibodies that react with hemoglobin in the blood. A positive FIT test result indicates blood in the stool sample and cannot provide a definitive cancer diagnosis. There may be many other reasons for blood in the sample, including ulcers, hemorrhoids, anal fissures, bacterial infection, etc. The test acts as a good screening tool.

Scans

Photo by CDC on Unsplash

An X-ray is a fast, cost-effective, painless scan. It offers images of the tissues in our body, including our bones. They help detect lung infections and signs of pneumonia or tuberculosis. 

X-rays involve very low radiation for a brief period and are considered safe. 

CT (Computerized Tomography)

A CT offers more detailed insights into the structures in our body compared to a traditional X-ray. In a CT, a beam of X-rays rotates around a patient, offering 2D image slices of the patient. Computers can combine all 2D images to provide a 3D visual of the patient, including the bones, organs, and tissues. 

A CT is used to detect infections in patients who are acutely ill. Using a CT scan, a healthcare provider can find the source of a bacterial infection based on abnormal fluid accumulation or tissue growth.

MRI (Magnetic Resonance Imaging)

MRIs use a magnetic field to get detailed images of our bodies. MRIs provide much more detailed insights into our body structures like organs and other tissues. For infectious diseases, an MRI is only used in acutely ill patients. MRI scans can detect infection and inflammation deep in our bones, brain, and other tissues. 

Meningitis, for example, is an inflammation of brain tissue. A CT or MRI scan can detect this infection but a spinal tap, blood culture, and a PCR may be needed to pinpoint the exact source of the viral or bacterial infection that caused the inflammation. 

Skin test: purified protein derivative (PPD) test

Photo by CDC on Unsplash

PPD tests help diagnose tuberculosis (TB) infections. TB is a serious bacterial infection caused by the bacteria Mycobacterium tuberculosis. The pathogen spreads through the air and can stay inactive in the body for years before showing signs of infection. 

The disease is so dangerous that the World Health Organization (WHO) calls TB “the world’s top infectious killer.” It is highly contagious, and people need to inhale very little amounts to get infected. People with weakened immune systems are at a higher risk of developing more severe symptoms of TB. 

The PPD test involves an injection of PPD (purified protein derivative) into the arm. People infected with TB will see swelling at the injection site after 2-3 days. No swelling or slight swelling is okay, but a bigger swelling (more than 10mm) may require medical intervention.

Key takeaways:

• There are many different kinds of tests to diagnose infectious diseases
• Laboratory tests are usually conducted on blood, urine, stool (feces), and tissue samples
• In some instances, a medical professional may perform a lumbar puncture (spinal tap) for cerebrospinal fluid, which is then analyzed in a lab
• Medical imaging scans can also help identify inflammation and infection to help diagnose infectious diseases. These include X-rays, CTs, and MRIs 
• PPD skin tests are standard screening tests for tuberculosis.

The GIDEON difference

GIDEON is one of the most well-known and comprehensive global databases for infectious diseases. Data is refreshed daily, and the GIDEON API allows medical professionals and researchers access to a continuous stream of data. Whether your research involves quantifying data, learning about specific microbes, or testing out differential diagnosis tools, GIDEON has you covered with a program that has met standards for excellence.

Learn more about diagnostic tests used to diagnose each infectious disease on the GIDEON platform.