Unit Converter
Procalcitonin (PCT)
(Highly Specific Biomarker for Bacterial Infection & Sepsis - Guides Antibiotic Use and Severity Assessment)
Synonyms
- Procalcitonin
- PCT
- Calcitonin precursor
- Prohormone of calcitonin
- Sepsis biomarker
Units of Measurement
- µg/L
- ng/L
- ng/mL
- ng/dL
- ng/100 mL
- ng%
Molecular Weight
~13 kDa
Unit Conversions
Mass relationships
1 ng/mL=1 µg/L1\ \text{ng/mL} = 1\ \text{µg/L}1 ng/mL=1 µg/L 1 ng/mL=100 ng/dL1\ \text{ng/mL} = 100\ \text{ng/dL}1 ng/mL=100 ng/dL 1 µg/L=1000 ng/L1\ \text{µg/L} = 1000\ \text{ng/L}1 µg/L=1000 ng/L \text{ng%} = \text{ng/dL}
Examples:
- 0.5 ng/mL = 0.5 µg/L
- 100 ng/L = 0.1 ng/mL
Description
Procalcitonin (PCT) is the prohormone of calcitonin, normally produced in the thyroid C-cells.
During systemic bacterial infection and sepsis, PCT is produced in large amounts by:
- Liver
- Lung
- Intestine
- Leukocytes
PCT rises 2–4 hours after infection and peaks by 12–24 hours, making it one of the most rapid and specific markers of bacterial sepsis.
It is not significantly increased in:
- Viral infections
- Autoimmune diseases
- Non-infectious inflammation
This makes PCT extremely useful for guiding antibiotic therapy.
Physiological Role
Under normal physiology:
- PCT → cleaved to calcitonin
- Levels are very low (<0.05 ng/mL)
During bacterial infection:
- Cytokines (IL-1β, TNF-α) + bacterial toxins cause unregulated PCT release.
PCT correlates with severity of bacterial infection.
Clinical Significance
HIGH Procalcitonin
1. Bacterial Sepsis (Primary Use)
Levels correlate with:
- Severity
- Organ dysfunction
- Mortality risk
2. Severe Bacterial Infections
- Sepsis & septic shock
- Pneumonia
- Meningitis
- Bacterial peritonitis
- Pyelonephritis
- Osteomyelitis
3. Post-surgical or Trauma Infections
4. Neonatal Sepsis Screening
Age-specific interpretation required.
5. Ventilator-associated pneumonia
Severity Interpretation
| PCT Level | Interpretation |
| <0.05 ng/mL | Normal |
| 0.05–0.5 ng/mL | Low risk of bacterial infection |
| >0.5 ng/mL | Suggests bacterial infection |
| >2.0 ng/mL | Strongly suggestive of sepsis |
| >10 ng/mL | High probability of septic shock |
LOW Procalcitonin
Seen in:
- Healthy individuals
- Viral infections
- Localized bacterial infections early in course
- Autoimmune diseases
- Chronic inflammatory conditions
Low PCT is especially useful to rule out bacterial infection.
Reference Intervals
(Tietz 8E + Mayo + ARUP + Surviving Sepsis Campaign)
Adults
- < 0.05 ng/mL
(= < 0.05 µg/L, < 50 ng/L)
Suspected Infection Thresholds
- > 0.25 ng/mL → consider bacterial infection
- > 0.5 ng/mL → likely bacterial infection
Sepsis Thresholds
- > 2 ng/mL → sepsis
- > 10–20 ng/mL → severe sepsis/septic shock
Neonates (first 72 hours)
Physiological rise → levels may reach 2–3 ng/mL normally.
Interpret with age-specific charts.
Diagnostic Uses
1. Diagnosis of Bacterial Infection
High sensitivity and specificity.
2. Sepsis Management
- Early detection
- Prognosis
- Monitoring severity
3. Guiding Antibiotic Therapy
Evidence-based threshold recommendations:
- PCT <0.25 ng/mL → stop/avoid antibiotics
- PCT >0.5 ng/mL → start/continue antibiotics
- Decrease of 80–90% from peak → consider stopping antibiotics
4. Differentiating Bacterial vs Viral
Very helpful in:
- Lower respiratory tract infections
- ICU patients
- Fever of unknown origin (FUO)
5. Postoperative Monitoring
Helps distinguish normal postoperative inflammation from infection.
Analytical Notes
- Immunoassays (chemiluminescent, ELISA)
- Not affected by mild hemolysis
- Renal failure can mildly elevate PCT
- Results affected by major trauma, burns, drug-induced shock
- Half-life ~24 hours
Clinical Pearls
- PCT rises much faster than CRP in bacterial infection.
- Viral infections → normal or mildly elevated PCT.
- Serial PCT levels are more important than a single value.
- In neonates, interpret with postnatal age due to physiologic peak.
- Renal dysfunction → slightly higher baseline PCT; trend still useful.
Interesting Fact
Procalcitonin production in sepsis is so widespread that almost every organ (not only thyroid C-cells) releases it, making it one of the most sensitive biomarkers of bacterial sepsis.
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Infection Markers
- Surviving Sepsis Campaign Guidelines
- IDSA Guidelines - Use of PCT in Respiratory Infections
- Mayo Clinic Laboratories - Procalcitonin
- ARUP Consult - Sepsis Biomarkers
- MedlinePlus / NIH - Procalcitonin Test