What it is
Procalcitonin is the 116-amino-acid precursor of the hormone calcitonin. The CALCA gene makes a 141-residue chain; the cell removes a 25-residue signal peptide, leaving procalcitonin. Cut further, it gives a 57-residue N-terminal piece, the 32-residue calcitonin and a 21-residue tail called katacalcin (UniProt P01258).
At 116 residues it is well over the 40-amino-acid line FDA uses to separate a peptide from a protein (21 CFR 600.3(h)(6)). So the honest description is a protein-sized precursor of a peptide hormone — the reason it sits in this library is calcitonin, not procalcitonin itself.
The same gene has a second life. Read with a different splice, CALCA produces α-CGRP, the neuropeptide behind migraine drugs (see CGRP). That is why assay makers test for interference from calcitonin, katacalcin and both CGRP forms: the 2017 KRYPTOR summary reports no interference from calcitonin up to 3.9 ng/mL, katacalcin up to 25.6 ng/mL and α- or β-CGRP up to 30 ng/mL.
Why a blood test measures it
In a healthy person almost all procalcitonin is processed inside the thyroid's C-cells, so blood levels are very low. In bacterial infection and sepsis many other tissues start releasing the uncut precursor, and the level climbs. That rise is what the tests read. Viral infections tend to produce a smaller rise — a pattern, not a rule, which is why every cleared use is phrased as an aid.
What the cleared tests say
Procalcitonin assays are Class II devices under 21 CFR 866.3215. The first clearance in FDA's database is B·R·A·H·M·S PCT LIA (K040887, 2005-01-07). The 2017 summary for B·R·A·H·M·S PCT sensitive KRYPTOR (K171338) lists four intended uses and the cut-offs attached to each:
| Setting (cleared intended use) | Cut-off as stated in the 510(k) summary | Associated with |
|---|---|---|
| ICU day 1, risk of progression | above 2.0 µg/L · below 0.5 µg/L | high risk · low risk of progression to severe sepsis or septic shock |
| Respiratory infection, starting antibiotics | below 0.10 · 0.10–0.25 · 0.26–0.50 · above 0.50 µg/L | antibiotics strongly discouraged · discouraged · encouraged · strongly encouraged |
| Respiratory infection, stopping | ≤0.25 µg/L, or a fall of more than 80% | antibiotics "may be discontinued" |
| Sepsis, stopping | ≤0.50 µg/L, or a fall of more than 80% | antibiotics "may be discontinued" |
| Severe sepsis, 28-day mortality | change from day 0 to day 4 (ΔPCT) | a fall of more than 80% marks lower risk |
These figures are reported as the labelling states them, with their settings attached. They are not reference ranges for an individual: the summary opens every use with "used in conjunction with other laboratory findings and clinical assessments", and a result belongs to the clinician who ordered it.
What it cannot tell on its own
The 2017 KRYPTOR summary (K171338) states these limits itself:
- It is not specific to bacteria. "Increased PCT levels may be observed in severe illness such as polytrauma, burns, major surgery, prolonged or cardiogenic shock."
- A low level does not rule infection out. Levels "may not be elevated" in infection with certain atypical pathogens, such as Chlamydophila pneumoniae and Mycoplasma pneumoniae.
- Kidney function matters. The severity of renal failure "may influence procalcitonin values" and is named as a potential confounder.
- Some groups were not analysed: the summary says PCT-guided therapy was not formally analysed for people under 18, pregnant women or immunocompromised people.
- Cut-offs are setting-specific. The respiratory discontinuation level (0.25 µg/L) differs from the sepsis one (0.50 µg/L) in the same document.
Where it stands
- A measured molecule, not a medicine. There is no approved procalcitonin drug, and it is not sold as a peptide product.
- Device record: 20 procalcitonin assay clearances or De Novo grants under the four PCT product codes in FDA's 510(k) database, from 2005-01-07 (B·R·A·H·M·S PCT LIA) to 2025-05-09 (DiaSys), filed by eleven companies including B·R·A·H·M·S, Roche, bioMérieux, Siemens, Beckman Coulter, DiaSorin and Ortho (openFDA, our count, 2026-10-14).
- Research record: PubMed indexes 13,018 records, 398 tagged as randomised trials (2026-10-14).
Related reading
The hormone cut from procalcitonin is calcitonin; the neuropeptide made from the same gene by another splice is CGRP. The library's other peptide that is mainly met as a blood test is BNP and NT-proBNP, and C-peptide is another precursor fragment measured in the lab.