Procalcitonin Blood Test Results and What They Mean

How Doctors Use a Procalcitonin Blood Test to Help Diagnose Sepsis

Doctor greeting patient in waiting room
Patient receiving test results. PhotoAlto/Frederic Cirou/Getty Images

Procalcitonin (PCT) is a blood test that is frequently performed if there is a suspicion that a patient may have bacterial sepsis, a severe systemic infection that can become life-threatening.The procalcitonin test is one way to make the diagnosis more quickly and potentially save lives.

Interpreting Procalcitonin Results

In a healthy individual, procalcitonin is unlikely to be elevated. If a person is sick, and sepsis is suspected, the procalcitonin lab would then be drawn to help determine if an infection is present.  It is important to note that procalcitonin does not diagnose a type of infection; rather, it indicates the likelihood that an infection may be present. If the procalcitonin test is positive, the physician then looks at additional tests and the patient's physical examination to determine if an infection is present, and how to treat it.

If sepsis is suspected, additional tests, such as blood cultures and a complete blood count (CBC), are often performed to determine if sepsis is present, and to determine what organism is causing the infection. In some cases, the procalcitonin is used to differentiate between a severe local infection, such as a respiratory infection, and an infection that has become systemic.

Local Infection Versus Systemic Infection (Sepsis)

A local infection is located in a specific part of the body; examples include a respiratory infection, a tooth infection or a bone infection. Sepsis is a systemic infection that has entered the bloodstream and is moving to all areas of the body.  While it is possible for a local infection to be life-threatening, sepsis is more likely to become severe and warrant major medical intervention. 

Procalcitonin Results

While the results of the procalcitonin test should be interpreted by a physician or another trained healthcare provider, the results are generally interpreted as follows:

  • Procalcitonin of 0-.05 µg/L is expected in a healthy individual.
  • Procalcitonin levels of less than .5 µg/L may indicate an infection, but such a level is unlikely to be a systemic infection like sepsis. A local infection, such as an infected tooth or respiratory infection, could be present.
  • A procalcitonin level between .5 - 2 µg/L indicates that a systemic infection is possible, but is not diagnostic of sepsis.
  • Procalcitonin levels of 2-10 µg/L indicate sepsis is likely.
  • Procalcitonin results of 10 µg/L or greater indicates a high likelihood of severe sepsis or septic shock and typically treatment with IV medication and fluid is initiated immediately.

The procalcitonin results can also be used to determine if an infection is likely to be caused by bacteria or a virus.  If an infection is known to be present, and the procalcitonin level is very low, the presence of bacteria is unlikely, and it is more likely that the infection is being caused by a virus.  This can prevent the use of antibiotics needlessly, as viruses are not treatable with antibiotic therapy. 

Other Causes of Elevated Procalcitonin

In addition to sepsis, there are additional reasons why an individual could have an elevated procalcitonin level. In newborns, for example, the normal level in the blood is high without an infection being present. Injuries--particularly serious trauma or surgery--can also cause a high result without the presence of sepsis. Some medications, shock not caused by sepsis, and even cancer can also cause an elevation in procalcitonin levels.

Conditions that decrease the level of oxygen available to the tissues of the body, regardless of the root cause, can cause an elevated procalcitonin when there is no infection present.  These can range from decreased oxygen intake due to asthma or pneumonia to cardiac arrest.

Low Procalcitonin Levels With or Without Sepsis

It is possible for a patient who has sepsis to present with a low procalcitonin level. If the test is done very early in the course of the illness, the levels may be low, but will likely increase over the next few hours or days.

Low levels may also be detected when no source of infection is clearly in evidence. This may be the case if an infection is difficult to locate. A patient may have no symptoms of infection when the test indicates an infection is present.

Procalcitonin and Sepsis: What Does It All Mean?

In essence, a procalcitonin level on its own only predicts a likelihood that a type of infection is present. The procalcitonin result is a guide, not a diagnosis. Think of the test result as a sign that further testing should be done, not an actual diagnosis.

Patients do not receive treatment based on a procalcitonin level alone: the entire scope of their current health is evaluated, and treatment is based on the clinical judgment of the physician. The patient is always evaluated for additional signs and symptoms of infection. For a patient who is examined early in the course of sepsis, and has a low procalcitonin level, repeating the test at a later time may reflect that sepsis is present. 

The true value of the procalcitonin level is that it can lead to earlier treatment of sepsis, which can quickly become life-threatening. For an individual who is developing sepsis, being treated a few hours faster because of an elevated procalcitonin level can make the difference between an illness that responds quickly to antibiotics and an illness that is severe enough to require ICU level care.

Sepsis is sometimes difficult to diagnose because the individual in question may not even realize that they have an infection.  Something as minor as athlete's foot can lead to sepsis. For these patients who are unaware of an infection, the procalcitonin test is particularly useful.

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