What it is (overview)

Procalcitonin (PCT) is a blood test that measures the level of procalcitonin, a protein that typically rises in the body during a bacterial infection. Because levels can increase quickly when a serious bacterial infection is present, procalcitonin is used as a biomarker to help doctors evaluate possible sepsis (a life-threatening response to infection) and to support decisions about antibiotic therapy, especially in critical care and emergency settings.

In plain language, the test helps answer questions like: “Is this illness more likely caused by bacteria?” and “How severe might the infection/inflammation be?” Higher procalcitonin levels generally suggest a stronger inflammatory response that is often due to bacterial infection. Lower levels make a significant bacterial infection less likely, which can help avoid unnecessary antibiotics in some situations.

It’s important to know that procalcitonin is one piece of the diagnostic puzzle. Results are interpreted together with symptoms, physical exam findings, vital signs, other blood tests (such as white blood cell count, CRP, lactate), imaging, and cultures. Some non-infectious conditions can also raise PCT, and not all infections cause a high procalcitonin level—so your clinician will use clinical context to guide decisions.

When & why it's usually done

Doctors commonly order a procalcitonin blood test when they need help determining whether symptoms are likely due to a bacterial infection, assessing the risk of sepsis, or deciding when to start, continue, or stop antibiotics. It may be used in the emergency department, hospital wards, or intensive care unit for people who appear significantly ill.

This diagnostic test may be considered if you have signs or symptoms such as:

• High fever or chills
• Fast heart rate, rapid breathing, or low blood pressure
• Confusion, extreme weakness, or sudden worsening of overall condition
• Shortness of breath, cough, or chest discomfort (concern for pneumonia)
• Severe abdominal pain, vomiting, or suspected intra-abdominal infection
• Burning with urination, back/flank pain, or suspected kidney infection
• Red, hot, painful skin or a wound that looks infected

It’s also often used when the diagnosis is uncertain—for example, when symptoms could be caused by a virus, bacteria, or non-infectious inflammation. In respiratory infections, procalcitonin can support antibiotic stewardship (using antibiotics only when helpful). In hospitalized patients, repeating the test over time may help monitor response to treatment and guide the duration of antibiotic therapy.

Common diseases related to it

• Sepsis and septic shock
• Bacteremia (bacteria in the bloodstream)
• Pneumonia (including community-acquired and hospital-acquired pneumonia)
• Urinary tract infection (UTI) and pyelonephritis (kidney infection)
• Meningitis (especially bacterial meningitis)
• Intra-abdominal infections (e.g., appendicitis, diverticulitis, peritonitis)
• Skin and soft tissue infections (e.g., cellulitis, infected wounds)
• Post-surgical infection or complications after major surgery
• Severe systemic inflammation (SIRS) where infection is a concern

Health goals where it may help

• Early detection and risk assessment of serious bacterial infection and sepsis
• Guiding antibiotic therapy (when antibiotics are likely to help and when they may be avoided)
• Monitoring response to treatment in hospitalized or critically ill patients
• Supporting antibiotic stewardship to reduce side effects, resistance, and unnecessary medication use
• Helping triage severity in emergency and critical care (identifying who may need closer monitoring or escalation of care)