Infection Related Blood Tests

Infection Related Blood Tests

Reviewed By:
Timothy Yarboro, M.D.

Summary

A blood test is a procedure in which a sample of blood is taken from a patient and analyzed for abnormalities. Blood tests can help diagnose or monitor various conditions or diseases, including infectious diseases.

Samples can be obtained from a vein puncture, skin (capillary) puncture or artery puncture. Blood tests can be performed on different parts of the blood, including:

  • Whole blood (blood that has not been separated into its many components)

  • Blood plasma (the liquid part of the blood)

  • Blood serum (plasma that has had the clotting agent removed)

  • Blood cells (the individual red blood cells, white blood cells and platelets)

Many types of blood tests may be used to diagnose or rule out illnesses. Different types of tests may be performed depending on the nature of illness that is suspected. Tests that are often associated with infectious diseases include complete blood counts (CBCs), differential white blood cell counts, antibody testing and blood cultures.

Some blood testing requires little or no preparation to achieve an accurate result. However, in other cases, patients may need to fast (avoid food and beverage intake for a certain length of time prior to the test) or avoid certain substances or activities prior to testing to ensure that certain factors do not skew test results.

Blood tests may be performed in a physician’s office, hospital, healthcare facility or laboratory. The exact steps taken during each blood test may be different, depending on the nature of the test.  In some cases, a sterile, sharp lancet (sharp, pointed instrument) is used for pricking the patient’s skin. When a substantial amount of blood is required, it is usually drawn from a vein in a process called a venipuncture or venous phlebotomy.

Although blood tests may help detect many diseases, additional tests are usually necessary to verify the diagnosis. It is important to note that blood tests cannot detect all conditions, and that a normal blood test does not necessarily indicate that a person is healthy.

Results from blood tests are usually returned to the requesting physician within 24 hours, depending on the type of blood test performed. Some tests, such as a blood culture, may require up to several days or longer before test results are known.

There are few risks or side effects associated with blood tests. Sometimes, bruising may occur at the site of injection. In rare cases, the vein used to obtain the sample may become inflamed (superficial phlebitis), or infection (e.g., cellulitis) may result from breaking the skin.

About blood tests

A blood test is a procedure in which a sample of blood is taken from a patient and analyzed for abnormalities. Physicians measure substances in the blood because abnormally high or low levels may affect normal body function. Blood tests can help diagnose or monitor levels of medications, various conditions or diseases, including infectious diseases.

For example, blood tests are often used to identify both common and uncommon bacterial, viral, fungal and parasitic agents (pathogens) that may be responsible for illness. A blood test known as a differential may be used to find elevated levels of white blood cells, which often indicates infection. Meanwhile, a type of test called a blood culture is frequently performed to identify the presence of certain bacterial or fungal pathogens in the blood.

Blood tests may be used to:

  • Determine a person’s risk of developing a disease
  • Look for changes in the body that make illness more likely
  • Screen for a disease before signs and symptoms appear
  • Help make a diagnosis
  • Rule out other diseases during diagnosis
  • Monitor the progression of a disease
  • Monitor the effectiveness of a patient’s treatment
  • Detect complications of a condition
  • Detect the spread of disease to other areas of the body
  • Detect recurrence of a disease

Samples of blood may be taken in one of several ways:

  • Venipuncture. A needle is inserted directly into a patient’s vein to draw blood into one or more tubes for laboratory analysis. Blood is usually drawn from a vein in the inner arm, opposite the elbow or elsewhere in the lower arm.

  • Skin capillary puncture. Used when only a small amount of blood is needed to perform the desired blood test. The skin of the fingertip, earlobe or heel (in infants) is pricked with a lancet (sharp, pointed instrument) to produce a drop of blood that is then collected for analysis.

  • Artery puncture. Used only when the blood test needs to be performed on a sample of oxygen–rich blood traveling from the heart to the body via the arteries. Blood is usually drawn from an artery in the wrist, but it may also be drawn from the groin.

Blood tests can be performed on different parts of the blood, including:

  • Whole blood (blood that has not been separated into its many components)

  • Blood plasma (the liquid part of the blood)

  • Blood serum (plasma that has had the clotting agent removed)

  • Blood cells (the individual red blood cells, white blood cells and platelets)

Certain blood tests are performed routinely to monitor chronic diseases such as diabetes. In addition, other blood tests, such as cholesterol tests, should be performed at least once a year for an overall assessment of a person’s health.

Although blood tests may help detect many diseases, additional tests are usually necessary to verify the diagnosis. The results of blood tests typically indicate the presence of illness or the need to monitor a potentially developing health problem.  However, it is important to note that blood tests cannot detect all conditions, and that a normal blood test does not necessarily indicate that a person is healthy.

Factors that may influence the results of blood tests include:

  • Adherence to preparatory instructions
  • Age
  • Medications and other drugs the patient is taking
  • Foods the patient consumes
  • Gender
  • General health
  • Medical history
  • Race
  • Variations in lab techniques

Common blood tests

Many types of tests may be used to diagnose or rule out illnesses. Different types of tests may be performed depending on the nature of illness that is suspected. For example, a complete blood count (CBC) and bacterial culture may be performed to confirm or rule out the presence of sepsis, a systemic manifestation of a bacterial infection that causes the body’s immune system to attack the body’s own organs and tissues. Sepsis can interfere with the normal functions of the body’s vital organs.

Common blood tests include:

  • Antibody tests. Blood tests are available to detect many different types of antibodies (substances produced by the body to fight specific types of infections). The presence of certain antibodies may indicate inflammation in the body, while other types of antibodies may indicate specific diseases. For example, there are particular antibodies that indicate the presence of certain specific infectious diseases such as HIV, yellow fever, cytomegalovirus infection or tapeworm infection, among many others.

  • Basic metabolic panel (BMP). A group of blood tests often ordered in hospital emergency rooms and in physician offices as part of a routine physical exam because the components provide important information about the status of a patient’s kidneys, electrolyte and acid/base balance. They also reveal a patient’s blood sugar (glucose) levels. Significant changes in these test results can indicate certain sudden and severe conditions, such as kidney failure, insulin shock or diabetic coma. These changes may also help diagnose the cause of respiratory distress or heart rhythm changes.

  • Blood culture. Test to determine if pathogens such as certain bacteria, mycobacteria or fungus are present in the blood. It is often performed if a patient experiences fever, chills or low blood pressure, and if an infection of the blood such as bacteremia or septicemia is suspected. When other conditions that may be suspected, performing a blood culture may assist in the diagnosis of disease such as meningitis, bone infections (osteomyelitis), inflammation of the heart’s lining (endocarditis), urinary tract infections, pneumonia and certain skin infections (e.g., cellulitis). When performing a blood culture, a sample of blood is obtained and incubated in a controlled environment for anywhere from one to seven days. If pathogens appear, further culturing may take place to identify the specific type of pathogen. Blood cultures are most often performed to identify bacteria so that appropriate antibiotics can be prescribed to treat the condition.

  • Complement. Measures the blood levels of a certain group of proteins that help destroy foreign substances in the body. This type of blood test is also useful in evaluating kidney damage and in monitoring diseases such as pneumonia and autoimmune diseases.

  • Complete blood count (CBC). Measures the number of red blood cells, white blood cells and platelets in a patient’s blood. It also detects the amount of hemoglobin (a substance that carries oxygen throughout the body) in the red blood cells and a number of other factors. Patients who take medications that affect their blood count must have periodic CBCs to monitor their condition.

  • Differential. Also called a differential white blood cell count (WBC), this test measures the levels of the type of white blood cells in a blood sample. White blood cells are the major infection-fighting cells in the body. This test also includes information about abnormal cell structure and the presence of immature cells (a shift to the left). Normal levels of total white blood cells are generally considered to be in the range of 4,500 to 10,000 white blood cells per microliter. High numbers of white blood cells (leukocytosis) may indicate the presence of a bacterial infection. Lower levels of white blood cells (leukopenia) may result from the use of certain medications or in response to certain diseases and viral infections. In addition, a low WBC indicates that a patient may be at an increased risk of infection. Either a very low or very high white blood count may indicate the presence of conditions such as leukemia.

  • Electrolyte panel. Measures the body’s main electrolytes (sodium, potassium, chloride) to help determine whether or not the body has a proper fluid balance. Patients with persistent diarrhea or vomiting are at risk of an electrolyte imbalance and dehydration and may receive electrolyte panel testing.

  • Enzyme tests. A group of blood tests that measure the levels of enzymes (proteins required for chemical reactions to take place in cells) in the blood. These tests assess how well the body’s systems are functioning and whether any tissue damage has occurred. These tests may be used to measure liver or pancreas function and can help identify medical conditions such as pancreatitis and hepatitis.

  • Erythrocyte sedimentation rate (sed rate). Measures the rate at which red blood cells (erythrocytes) separate from the liquid part of blood (plasma) and fall to the bottom of a test tube, forming sediment. Higher sed rates indicate the presence of inflammation somewhere in the body.

  • Waste product test. A group of blood tests that measure the levels of specific waste products in a patient’s blood sample. They commonly test for uric acid, creatinine and blood urea nitrogen (BUN). When the kidneys are not working properly, waste products can build up in the body.

  • White blood cell scan. In this procedure, blood is withdrawn from a vein and white blood cells are separated from the blood sample. The cells are mixed (tagged) with a small amount of a radioisotope (radioactive material) and injected back into the body. Six to 24 hours later, the body is scanned to detect the accumulation of radiation in a manner that allows experts to view an image that reveals otherwise hidden areas of infection or inflammation. This test may be used to find problems that can be difficult to diagnose, such as certain abdominal conditions, or to detect disorders such as an abscess, osteomyelitis, or pyelonephritis (inflammation of the kidney). A white blood cell scan may also be used in patients with unexplained fever.

Before the blood test

Certain blood tests require little or no preparation for accurate results. For some tests, patients may need to fast (avoid food or beverage intake for a certain length of time prior to the test) or avoid certain substances prior to testing. This ensures that certain factors do not skew test results. For example, patients may need to reduce or stop the use of certain medications (including herbal remedies) and/or vitamins at some point prior to the blood test.

In addition, exercise, alcohol and caffeine consumption may be temporarily restricted or suspended prior to a blood test. Other factors known to potentially alter the results of some blood tests include dehydration or excessive eating.

Patients should report engaging in any activities that deviate from their pre-testing regimen. Honesty is important – failure to closely follow a physician’s preparatory recommendations can lead to inaccurate test results. This can lead to ineffective or unnecessary treatments and unwarranted additional tests and expenses.

During the blood test

Blood tests may be performed in a physician’s office, hospital, healthcare facility or laboratory. On the day of the test, the procedure will be explained and patients will have the opportunity to ask questions. To determine if a patient is taking any medications that will interfere with the test’s accuracy or has any history of clotting problems, a healthcare professional may also ask questions about the patient’s medical history prior to administering the test.

Having blood drawn for a blood test is a simple and nearly painless process for most people. If the test only requires a drop or two of blood, a prick of the finger, earlobe or heel (in infants) can provide enough blood for testing.

During a skin capillary puncture test, a sterile lancet (a sharp, pointed instrument) is used for pricking the patient’s skin. The punctured area is then gently squeezed to extract drops of blood that are collected in tiny glass tubes. To stop the bleeding, sterile gauze is applied with light pressure to the punctured area. A bandage is usually not necessary since bleeding is minimal in most cases.

When a substantial amount of blood is required, it is usually drawn from a vein in a process called venipuncture. First, the area is cleaned with rubbing alcohol – usually the inside arm, opposite the elbow. Then, a wide elastic band or piece of latex tubing may be tightened around the upper arm to slightly increase pressure in the vein. Next, one end of a sterile, double-ended needle that has been attached to an open-ended syringe (containing an empty test tube) is inserted into the vein. Because the test tube creates a vacuum effect, blood flows directly from the vein through the double-ended needle and into the test tube. If blood must be taken from an artery instead of a vein, it is usually drawn from a small artery located on the inside or the top of the wrist.

The precise amount of blood to be drawn is determined by the type and number of tests scheduled. Usually, it is around one-fourth of an ounce (7 milliliters) for each receptacle. The test tubes may be changed more than once during venipuncture to either allow for more blood to be collected or to change the type of tubing being used. Tubes are marked with different color tops to indicate the way in which the collected blood will be preserved, such as:

  • A lavender top indicates that the tube contains an anticoagulant, which prevents the collected blood sample from clotting.

  • A red top indicates that the tube contains no anticoagulants, allowing serum and the blood to form a blood clot.

  • A gray top indicates that the tube contains a preservative, which prevents glucose (blood sugar) from breaking down in the tube.

After the necessary amount of blood is drawn, the needle is withdrawn and a small cotton ball or pad is applied with light pressure over the puncture site. After several minutes, the cotton will be discarded or replaced, and a small bandage will be placed on the puncture wound. The whole process usually takes less than 10 minutes. Bruising and soreness sometimes occur after blood is drawn. Typically, this is not a cause for concern. If soreness is present, patients can immediately apply a warm compress to the puncture site and repeat the application every three hours until the discoloration or pain subsides.

After the blood test

Following the withdrawal of blood, patients may resume medications and food intake according to their physician’s instructions. Immediately after the blood sample is taken, the test tubes are labeled with the date and the patient’s name and sent to the laboratory for testing. If a blood culture is performed, a sample of blood is obtained and incubated in a controlled environment to see if organisms appear.

Results from blood tests are usually returned within 24 hours, depending on the tests performed. Some tests, such as a blood culture, may require up to a week or longer before test results are known. Based on the results of the initial blood tests, additional blood tests may be ordered.

Blood test results are evaluated in relation to the “normal range” for that test. The range of values considered to be normal is the range of test results from the blood of normal, active, healthy people. It is usually adjusted for age and/or gender. When a person has a disease or health problem, their blood test results may be higher or lower than normal – or “outside of the normal range.” When a physician sees that a blood test is outside of the normal range, the test may be repeated to verify results or additional tests may be ordered to determine the underlying cause(s) of the abnormality.

Normal ranges for some tests may vary slightly from lab to lab, especially between labs that use machines to perform blood tests and those in which blood tests are performed by hand. Frequently, results of a patient’s blood test are compared to another “known” blood sample taken from a healthy individual that is run at the same time and is designated as the “normal control.” When the “normal control” sample falls within the normal range, or reaches a specific “known” measurement, the laboratory results confirm that the blood test has been carried out accurately.

Although blood tests are useful in the diagnosis of many conditions or diseases, other diagnostic tests are usually performed to confirm the results. These may include imaging tests such as x-rays, bone scans or computed axial tomography (CAT) scans, urine tests, stool tests, biopsies, samples of other bodily fluids and other procedures.

Once the source of a patient’s illness has been identified, treatments can begin. For bacterial or fungal infections, treatment often is focused on antibiotic or antifungal medications. In some cases, patients who are being tested using a blood culture may begin preliminary antibiotic treatment before the results of the blood culture are known.

Potential risks with blood tests

There are few risks or side effects associated with blood tests. Sometimes, bruising may occur at the site of injection. This can be minimized and alleviated by keeping direct pressure on the spot for several minutes after the needle has been removed.

In rare cases, the vein used to obtain the sample may become inflamed. Applying warm compresses to the spot several times daily can help reduce swelling. Blood tests also may cause infection in rare cases.

People with bleeding disorders or who are taking anticoagulants (including aspirin) may have problems with continued bleeding. Those who experience these side effects should consult their physician.

Questions for your doctor regarding blood tests

Preparing questions in advance can help patients to have more meaningful discussions with their physicians regarding their healthcare. Patients may wish to ask their doctor the following questions related to blood tests:

  1. What type of blood test will you be performing?
  2. What conditions can you detect with this blood test? Is this what you suspect I have?
  3. Can I eat before the test or do I have to fast for a certain period of time beforehand?
  4. How else should I prepare for the test?
  5. Will the test be painful?
  6. How many vials of my blood are needed for this test?
  7. Are there any significant risks associated with the test?
  8. Can the test have any side effects that I should report immediately to you, such as swelling at the injection site?
  9. How long will I have to wait for test results?
  10. Will I need additional diagnostic testing to confirm the blood test results? What type?
Scroll to Top