How the Procalcitonin Test Diagnoses Sepsis

Identifying Sepsis Before It Turns Deadly

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

what is a procalcitonin blood test

Verywell / Laura Porter

Role of Procalcitonin

Procalcitonin is a peptide naturally produced in the body that serves a marker for sepsis. In a healthy individual, procalcitonin is unlikely to be elevated. If a person is sick, and sepsis is suspected, the procalcitonin test would be ordered to determine if there is a widespread dissemination of the bacterial infection. 

It is important to note that procalcitonin does not diagnose a type of infection; rather, it is a strong indicator of a systemic infection. If the procalcitonin test is positive, the physician will then order additional tests to confirm the diagnosis. These may include blood cultures and a complete blood count (CBC).

Sepsis is the leading cause of death in U.S. hospitals, according to the San Diego-based Sepsis Alliance. Moreover, 19 percent of people with sepsis will be rehospitalized within 30 days as a result of the aftermath of the infection.

Local vs. Systemic Infections

In some cases, a procalcitonin test will be used to differentiate between a severe local infection and a systemic one. Examples of a local infection include pneumonia, a tooth abscess, or a urinary tract infection (UTI). Sepsis, by contrast, 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 become life-threatening, sepsis is more likely to be severe and warrant aggressive medical intervention. Sepsis is sometimes difficult to diagnose because it is not always symptomatic in the early stages. Oftentimes, something as minor as an athlete's foot can lead to sepsis.

Symptoms of sepsis include fever, difficulty breathing, low blood pressure, and confusion. If left untreated, sepsis can lead to septic shock and death. In fact, the risk of death increases by no less than 8% for every hour that treatment is delayed.

Interpreting the Test Results

While the results of the procalcitonin test should be interpreted by a doctor or qualified health provider, the results are broadly classified as follows:

  • Normal: 0 to 0.05 micrograms per liter (µg/L)
  • Low risk of sepsis: less than 0.5 µg/L
  • Possible sepsis: between 0.5 µg/L and 2 µg/L
  • Moderate to high risk of sepsis: 2 µg/L and 10 µg/L
  • Severe sepsis: 10 µg/L or greater

The procalcitonin results can also be used to determine if an infection is likely to be caused by bacteria or a virus. If infection symptoms are severe but the procalcitonin level is low, the cause is more likely viral. This can ensure the appropriate drug therapy and avoid the needless use of antibiotics.

Studies have shown that, in people with sepsis, higher procalcitonin levels are associated with a greater risk of progression to severe sepsis, septic shock, and death.

Causes of High Procalcitonin

In addition to sepsis, there are additional reasons why an individual could have an elevated procalcitonin level. In newborns, for example, the procalcitonin level is naturally high.

Serious injuries or surgery can also cause an elevation without the presence of sepsis. Non-septic shock, medications, and cancer can also cause trigger an elevation.

Any condition that decreases the blood oxygen level, irrespective of cause, can cause an elevated procalcitonin even when no infection is present. These include problems like asthma, pneumonia, and cardiac arrest.

Low Procalcitonin With Sepsis

It is possible for the test to show a low procalcitonin level in the very early stages of sepsis, but it will likely increase over the next few hours or days.

By contrast, a person may have high levels in the early stages of sepsis but have no symptoms at all. Almost invariably, symptoms will manifest once the bacterial infection (bacteremia) begins to affect different organ systems.

A Word From Verywell

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.

People 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 true value of the procalcitonin level is that it can lead to earlier treatment of sepsis. Treating the infection even a few hours earlier can make the difference between an illness that responds to antibiotics and one that lands you in intensive care.

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