Procalcitonin test

No special preparation is needed. A healthcare professional will take a blood sample from a vein in your arm. Because this test is usually done when someone is acutely unwell, it is often taken alongside other urgent blood tests at the same time.

What the test measures 

Procalcitonin is a substance produced as part of the process of making calcitonin, a hormone released by the thyroid gland. In healthy people, only a very small amount of procalcitonin is present in the blood. 

When the body is fighting a widespread bacterial infection, many cells across the body begin producing procalcitonin in large amounts. Blood levels can rise rapidly — within hours of a serious infection starting. Viral infections, by contrast, do not usually cause the same sharp rise. This difference is what makes procalcitonin useful in helping to distinguish bacterial from non-bacterial causes of serious illness. 

PCT levels also rise in some non-infectious conditions, including severe trauma, major surgery, pancreatitis, serious burns, heart attack, and acute rejection of a transplanted organ. Your clinical team will always interpret your result in the context of your full clinical picture. 

What your results mean 

PCT results are interpreted alongside your symptoms, other blood tests, and your overall clinical condition. No single result should be read in isolation. 

If your PCT level is low 

A low procalcitonin level in a seriously ill person suggests that severe bacterial infection (sepsis) is less likely. It indicates a lower risk of progression to severe sepsis or septic shock. 

However, a normal result does not completely rule out sepsis. PCT levels may still be low if: 

• the infection is in the early stages — PCT may not yet have risen if the infection started fewer than six hours ago 
• the infection is localised to one part of the body and has not spread into the bloodstream 
• the cause of the illness is a viral infection, transplant rejection, or another non-bacterial condition 

Your clinical team will not rely on PCT alone to rule out sepsis. All your test results and symptoms will be considered together. 

If your PCT level is moderately raised 

A moderate rise in procalcitonin may be caused by a non-infectious condition — such as recent surgery, trauma, or pancreatitis — or by an early bacterial infection. Your clinical team will review this result carefully alongside your other findings before making any decisions. 

If your PCT level is high 

A significantly raised procalcitonin level indicates a high likelihood of sepsis caused by a widespread bacterial infection. It also suggests a higher risk of the illness becoming more severe, including the risk of septic shock. 

A high PCT result will prompt your clinical team to begin or continue antibiotic treatment and monitor you closely. 

If you are being monitored during treatment 

Procalcitonin is also used to track how well treatment is working: 

• Falling PCT levels — suggest that the infection is responding to antibiotic treatment. Your clinical team may use this as a guide to safely stop antibiotics earlier than would otherwise be the case, reducing the risk of antibiotic resistance. 
• PCT levels that stay high or continue to rise — suggest that the current treatment may not be working and a change of approach may be needed.

If your procalcitonin is low and your clinical team consider sepsis unlikely, they will continue to investigate other possible causes of your symptoms. Treatment will be guided by the full picture of your condition. 

If your procalcitonin is raised and sepsis is suspected, your clinical team will act quickly. This may include starting antibiotics, giving fluids through a drip, and monitoring your organs closely. PCT will usually be measured again over the following days to track your response. 

Because antibiotic resistance is a growing concern, your clinical team may use your falling PCT levels as a guide to stop antibiotics safely once the infection has been treated — avoiding unnecessary antibiotic use. Your doctor will work with you and your care team to make this decision.

• Timing of the test: PCT may not yet have risen if blood is taken within the first six hours of an infection starting. A low early result does not rule out developing sepsis, and the test may be repeated. 
• Non-infectious causes: PCT can rise in conditions that are not infections, including major surgery, serious trauma, severe burns, heart attack, pancreatitis, and acute organ transplant rejection. Your clinical team will account for these possibilities. 
• Viral infections: viral infections generally do not cause the same rise in PCT as bacterial infections. This helps PCT distinguish between bacterial and viral illness, though it is not a perfect test. 
• Localised infection: a bacterial infection confined to one area of the body — such as a skin infection or urinary tract infection that has not spread — may not cause a significant rise in PCT. 
• Kidney function: reduced kidney function can affect how the body clears procalcitonin, which may influence results. 
• Test availability: PCT testing is not available in all UK laboratories. Where it is not available, other markers such as CRP and white blood cell count are used to guide clinical decisions. 

Procalcitonin is never used alone. It is always considered alongside other tests. Your clinical team may also request: 

• C‑reactive protein (CRP): another marker of inflammation and infection. CRP also rises with bacterial infection but more slowly than PCT and is less specific for severe systemic infection. 
• Full blood count (FBC): looks at the number and type of blood cells. A raised white blood cell count can suggest infection or inflammation. 
• Blood cultures: blood is tested to see whether bacteria are growing in the bloodstream. A positive blood culture confirms bacteraemia (bacteria in the blood) and can identify which bacteria is causing the infection, guiding antibiotic choice. 
• Lactate: a raised lactate level is a sign that the body’s tissues are not getting enough oxygen — an important marker of severity in sepsis. 
• Urea and electrolytes (U&E) and creatinine: used to assess kidney function, which can be affected by sepsis. 
• Liver function tests (LFTs): to check whether the liver is being affected. 
• Cerebrospinal fluid (CSF) analysis: if bacterial meningitis is suspected, a lumbar puncture may be performed to test the fluid around the brain and spinal cord. 

Sepsis is a life-threatening reaction to an infection. It happens when the immune system responds to infection in a way that starts to damage the body’s own tissues and organs. Sepsis is a medical emergency. Early treatment with antibiotics and fluids greatly improves outcomes. For more information, visit sepsis (NHS).