Procalcitonin (PCT)

A protein that rises in serious bacterial infections. Clinicians use it with other findings, mostly in hospital, to judge severity and guide treatment.

AI-verified against cited sources. Awaiting expert reviewAlso called PCTLOINC 33959-8

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What it is

Procalcitonin (PCT) is the inactive precursor of calcitonin, a hormone made in the thyroid gland. In healthy people the blood level is very low. In some serious infections, especially bacterial ones, many tissues release large amounts of it within hours. Viral infections usually raise it much less. The test is used mostly in hospitals and emergency departments.

Why it matters

A very low level makes a widespread bacterial infection or sepsis less likely, and a high or rising level makes it more likely. Clinicians weigh it together with your symptoms, exam and other tests. In some hospitals a falling level is one input the team uses when deciding whether antibiotics can stop. The 2021 Surviving Sepsis Campaign guideline suggests using procalcitonin with clinical judgment to help decide when to stop antibiotics in adults with sepsis, but suggests against using it to decide when to start them. It cannot diagnose an infection on its own.

If it’s high

A level above the usual range means the body is reacting to something. Higher numbers mean a stronger reaction, but the number does not show the cause. The most common reason in hospital is a serious bacterial infection, but surgery, major injury and other physical stress can raise it too. Hospitals use different decision points. Levels below about 0.5 ng/mL make sepsis less likely, levels of about 0.5 ng/mL and above are often read as suggesting sepsis is possible, and levels above 2 ng/mL as a sign of a high risk of organ strain from the body's inflammatory response.

Common causes

A serious bacterial infection that has spread through the body (sepsis)
Recent major surgery, serious injury or burns
Poor blood flow from severe heart or circulation problems (circulatory shock)
Severe pancreatitis
Some autoimmune or inflammatory conditions
Medullary thyroid cancer (rare)

What can help

Ask your care team what they think is driving the number and which other results they are weighing
Ask whether the test will be repeated to show the trend, since the direction often matters more than one value

Usual ranges

What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.

For adults: 0–0.24 ng/mL. Source: Mayo Clinic Laboratories (catalog page mirrored on testcatalog.org), Annals of Laboratory Medicine.

Adults (this one applies)0–0.24 ng/mL
Sources: Mayo Clinic Laboratories (catalog page mirrored on testcatalog.org), Annals of Laboratory Medicine

What can affect your result

Timing: levels take a few hours to start rising and peak after a day or two, so an early sample can read low
Where the infection is: infections confined to one area may not raise it
Non-infectious stress on the body, such as surgery, trauma, burns or pancreatitis
Kidney function: clearance is slower in severe kidney problems, although it does not build up over time
Age: adult interpretation does not apply to newborns and infants
The assay: different assays agree well at the commonly used decision points, but each lab sets its own cutoff
A sample taken very early in an infection can read low, because levels start to rise only 2 to 4 hours after the trigger.
Infections confined to one spot, such as an abscess, empyema or mediastinitis, may not raise it much.
Major surgery, severe trauma, burns, severe pancreatitis, heat stroke and prolonged circulatory shock can raise it without a bacterial infection.
Severely reduced kidney function slows how fast it clears, although it does not make it build up over time.
Medullary thyroid cancer can raise it.

Starts rising about 2 to 4 hours after the trigger, peaks at 24 to 48 hours, and falls by about half each day or so (half-life roughly 25 to 36 hours) once the trigger clears.

When to talk to a clinician

Procalcitonin is usually ordered in hospital or urgent care, where the team reviews it right away. If you have a result above 2 ng/mL outside the hospital, or your lab flagged it, call the clinician who ordered it the same day. Call 911 or your local emergency number for fever with confusion, trouble breathing, a very fast heartbeat, clammy skin or fainting, whatever the result.

Questions you could ask

How will you use this result together with my symptoms and other tests?
If my result is low and I still feel unwell, what should I watch for?
What do you think is causing this result, and what else are you looking at?
Could my recent surgery or injury explain part of it?
Should we repeat it to see which way it is heading?

Questions people ask

Does a normal procalcitonin rule out an infection?

No. A low level makes a serious infection that has spread through the body less likely, but an early sample or an infection in one spot, such as an abscess, may not raise it. Your clinician looks at the whole picture and may repeat the test.

Why are there different cutoffs for procalcitonin?

Hospitals and studies use different decision points for different questions, mainly between about 0.1 and 0.5 ng/mL for respiratory infections and 0.5 to 2 ng/mL or higher for sepsis. Use the range and flags on your own report, and ask your clinician how they are using the number.

How fast does it change?

It starts to rise within a few hours of a trigger, peaks after about one to two days, and then falls by about half each day or so once the problem is under control. That is why the trend over several days is often more useful than a single value.

Show the science and sources

Units

1 ng/mL = 1 ng/mL
1 ng/mL = 1 µg/L

For clinicians and researchers

PCT is a prohormone of calcitonin (114 to 116 amino acids). It is made in thyroid C-cells in health, and in infection many tissues release it, driven by cytokines released in response to bacterial infection. Interferon-gamma, released in viral infection, blunts its production. Induction lags 2 to 4 hours after onset, levels peak at 24 to 48 hours, and the half-life is about 25 to 30 hours (Rhee 2017); Meisner (2014) puts the 50% disappearance time at 1 to 1.5 days and notes that clearance is slower, but without accumulation, in severe renal dysfunction. Healthy values are under 0.1 ng/mL (Meisner), while Mayo Clinic Laboratories prints 0.00 to 0.24 ng/mL; Meisner calls 0.2 ng/mL or below a useful rule-out level, 0.5 ng/mL or above abnormal, and above 2 and above 10 ng/mL signs of high and very high risk of organ dysfunction. Trial algorithms in respiratory infection used 0.1, 0.25 and 0.5 µg/L as decision points, and ICU trials used 0.5 µg/L or an 80% fall from peak to guide stopping antibiotics (Rhee 2017). The 2021 Surviving Sepsis Campaign guideline issued a weak recommendation against using PCT to guide starting antimicrobials and a weak recommendation for using it with clinical evaluation to guide stopping them in adults with sepsis and adequate source control. A meta-analysis it cites reported pooled sensitivity of 77% and specificity of 79% for sepsis in critically ill patients. False positives follow major surgery, trauma, burns, cardiogenic shock, heat stroke and pancreatitis. False negatives occur with localized infection and very early sampling. Several assays exist and agree well at clinically relevant cutoffs (Rhee 2017). The FDA first cleared it as a sepsis aid in 2005 and for serial use to assess progression and 28-day mortality risk in 2016.

LOINC 33959-8 (primary), 75241-0, 111765-4. Specimen: serum-or-plasma.

Sources

Procalcitonin, Serum (Mayo test ID PRCAL), Mayo Clinic Laboratories (catalog page mirrored on testcatalog.org)
Update on Procalcitonin Measurements, Annals of Laboratory Medicine (2014)
Using Procalcitonin to Guide Antibiotic Therapy, Open Forum Infectious Diseases (2017)
Surviving Sepsis Campaign: International Guidelines for Management of Sepsis and Septic Shock 2021, Intensive Care Medicine / Critical Care Medicine (2021)
Procalcitonin Doubling Time for Postoperative Monitoring of Medullary Thyroid Carcinoma: A Comparative Kinetic Analysis, Clinical Endocrinology (2026)

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.