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What is a procalcitonin test for antibiotics?

Procalcitonin (PCT) is a biomarker that can be used to guide antibiotic treatment decisions. PCT levels rise in response to bacterial infections, so PCT testing can help determine whether antibiotics are likely to be beneficial or if antibiotic therapy can be reduced or stopped.

How does procalcitonin guide antibiotic use?

During bacterial infections, levels of procalcitonin rise rapidly. Higher levels of PCT indicate a greater likelihood that antibiotics will be helpful. Conversely, low or normal PCT levels suggest the infection is more likely viral or not severe, and antibiotics may not be warranted.

Thus, PCT levels can assist clinicians in deciding whether to:

  • Start or withhold antibiotic therapy
  • Continue or discontinue antibiotic treatment
  • Switch from intravenous to oral antibiotic therapy
  • Shorten the duration of antibiotic therapy

Regularly monitoring PCT levels in hospitalized patients allows doctors to re-evaluate the need for antibiotics frequently based on objective data. This helps avoid unnecessary antibiotic use and reduce the risk of antibiotic resistance.

How is the procalcitonin test performed?

A procalcitonin test requires collecting a blood sample through venipuncture (inserting a needle into a vein, usually in the arm). The blood is then processed by a laboratory to measure the level of PCT using an immunoassay technique. Results are usually available within several hours.

When should procalcitonin be tested?

Procalcitonin testing is most useful in the following situations:

  • At hospital admission in cases of suspected serious infection
  • During hospitalization to monitor infection and guide continued antibiotic therapy
  • In critically ill patients in the ICU to determine if infection is the cause of deterioration
  • To differentiate bacterial vs. viral meningitis
  • To identify post-operative infections after surgery

Frequent PCT measurement is key for utilizing it to reduce excessive antibiotic exposure. Testing every 2-3 days guides clinicians in stopping antibiotics when PCT levels drop.

Procalcitonin levels and interpretation

Normal PCT levels in healthy individuals are very low, below 0.10 ng/mL. Levels above 0.25 ng/mL are generally indicative of bacterial infection. Higher levels correlate with greater infection severity and higher mortality risk. However, some caveats exist:

  • Levels may be elevated after trauma or surgery initially, even without infection.
  • Levels rise within 2-4 hours of infection and peak at 24-48 hours.
  • Very high levels (>10 ng/mL) indicate severe sepsis or septic shock.
  • Slowly declining levels signal clinical improvement.
  • PCT cannot differentiate between types of bacterial infection.

The following table summarizes the interpretation of different procalcitonin levels:

PCT Level Interpretation
Less than 0.10 ng/mL Normal PCT level. Bacterial infection unlikely.
0.10 – 0.25 ng/mL Suggests low possibility of bacterial infection. Consider withholding antibiotics.
0.25 – 0.50 ng/mL Bacterial infection likely. Initiate or continue antibiotics.
0.50 – 1.00 ng/mL Bacterial infection probable. Prescribe antibiotics.
1.00 – 10.00 ng/mL Significant bacterial infection present. Treat with antibiotics.
Above 10.00 ng/mL Severe sepsis or septic shock probable. Urgent antibiotic therapy indicated.

Benefits of procalcitonin monitoring

Numerous studies have demonstrated that using PCT protocols to guide antibiotic management in hospitals can:

  • Reduce total antibiotic exposure by 20-40%
  • Shorten antibiotic duration by 2-4 days on average
  • Lower risk of antibiotic-related side effects
  • Increase adherence to antibiotic stewardship programs
  • Improve clinical outcomes in sepsis and pneumonia
  • Cut antibiotic costs and combat antibiotic resistance

Despite the evidence, procalcitonin testing has not been widely adopted in many hospitals yet. But this is changing as more facilities make efforts to optimize antibiotic prescribing.

Limitations of procalcitonin testing

While procalcitonin levels provide useful information about bacterial infections, some limitations exist:

  • Cannot distinguish between types of bacterial infections
  • Other factors like trauma, surgery, severe inflammation can elevate PCT
  • Some infections, like endocarditis, may not raise PCT substantially
  • Frequent blood draws are required to monitor trends
  • Assays can be associated with false positive or negative results
  • Results should always be considered in clinical context

Procalcitonin is best utilized as one piece of information to aid clinical decision-making about antibiotics. It should not override physician judgment.

Who should have procalcitonin monitoring?

Procalcitonin testing is most useful in the following patient groups:

  • Critically ill patients – those with sepsis, shock, pneumonia, trauma, burns
  • Post-operative patients – especially orthopedic, cardiac, transplant surgery
  • Hospitalized patients with respiratory or urinary tract infections
  • ICU patients on mechanical ventilation
  • Immunocompromised patients – transplant, cancer, on immunosuppressants
  • Patients with bacterial meningitis – to differentiate from viral meningitis

Outpatients with mild infections generally do not require PCT testing. The costs may outweigh benefits in low risk ambulatory patients.

Procalcitonin versus other biomarkers

Procalcitonin has advantages over other biomarkers for bacterial infection:

Biomarker Pros Cons
Procalcitonin (PCT)
  • Rises rapidly in response to bacteria
  • Short half-life – reflects infection in real-time
  • High specificity for bacterial infections
  • Distinguishes bacterial from viral infections
  • Not a first-line diagnostic test
  • Expensive assay
  • Levels affected by non-infectious inflammation
C-Reactive Protein (CRP)
  • Widely available inexpensive test
  • Non-specific moderate rise with inflammation/infection
  • Lags behind PCT in response time
  • Less specific for bacterial infections
  • Difficult to interpret due to nonspecific elevations
White Blood Cell Count
  • Simple inexpensive test
  • Very non-specific indicator of infection/inflammation
  • Poor sensitivity and specificity
  • Poor correlation with infection severity
  • Very nonspecific – many false positives

While more expensive, PCT provides superior specificity for bacterial infection compared to other commonly used biomarkers.

Procalcitonin versus other infection indicators

Procalcitonin levels complement other indicators of infection that clinicians consider:

  • Vital signs – fever, heart rate, blood pressure
  • White blood cell count – elevated in most infections
  • Microbiologic cultures – positive blood, urine, or other cultures definitively indicate infection and guide antibiotic selection
  • Radiologic imaging – CT scans, x-rays can detect infectious sources
  • Clinical status – overall appearance, mental status, organ dysfunction

Procalcitonin testing should always be interpreted in the full clinical context, not used in isolation. However, PCT provides a more specific marker of bacterial infection severity compared to other parameters.


Procalcitonin is emerging as a valuable biomarker to improve antibiotic stewardship in hospitals. Regular PCT measurement can help clinicians determine when to start, stop, or narrow antibiotic therapy based on objective data. This promotes judicious antibiotic use, reducing unnecessary exposure and combating resistance.

While limitations exist, procalcitonin testing has been shown to safely decrease antibiotic duration and improve outcomes across diverse patient populations. Wider adoption of PCT-guided antibiotic protocols has the potential to transform antibiotic prescribing patterns in hospitals and positively impact public health through preserved antibiotic efficacy.