Calcitonin gene-related peptide 1
The CALCA gene at 11p15.2-p15.1 codes for several different active peptides that are differently expressed in various tissues (eg as the 32 amino acid calcitonin at 85 – 116, calcitonin gene-related peptide 1 and the 21 amino acid katacalcin at 121 – 141). ProCGRP a 128 amino acid peptide coded by the CALCA gene that is further processed to the 37 amino acid calcitonin gene-related peptide 1 (amino acids 83 – 119, alpha GGRP) This has three isoforms formed by alternative splicing and is about 50% similar to the 37 amino acid product of the adjacent gene CALCB.
Calcitonin, a hormone is just one active peptide coded by CALCA , a gene at 11p15.2-p15.1 with 6 exons although the first exon is not apparently translated. The first 3 exons of CALCA are present in both calcitonin and CGRP mRNA, although exon 1 is not translated. Exon 4 contains the calcitonin-coding sequence. Exon 5 encodes the CGRP sequence. The peptides produced include:
- Pro-calcitonin (calcitonin precursor) is a 141 amino acid peptide that is normally produced as part of the process of producing:
- Three potential isoforms
- Amino acids 85-116 of procalcalcitonin
- Katacalcin (calcitonin carboxyl-terminal peptide, CCP, PDN-21)
- Amino acids 121-141 of procalcalcitonin
- In the presence of some inflammatory conditions, however, it can be produced by virtually all cells - most of which do not have the capacity to use it to convert it to calcitonin. It can therefore be used, not only as an inflammatory marker, but also as an indicator that the patient is more likely to have a bacterial infection.
- ProCGRP a 128 amino acid peptide
Raised procalcitonin is one of the newer criterion for systemic inflammatory response syndrome.
Use as a marker for bacterial infection
Unhappily its effectiveness in this indication is no more effective than best current antimicrobial practice. It has been used, along with other markers, in studies in which its use has reduced considerably the proportion of patients in whom antibiotics have been used.. However in real life practice this promise with respect to better antimicrobial stewardship has failed. This may be because in real life clinicians do not trust the test as much and the time delay in test processing is unacceptable for busy patients and clinicians. More importantly perhaps antimicrobial stewardship has advanced since the original mandatory RCTs and low procalcium levels in a patient in whom a the clinician would not prescribe an antibiotic anyway do not add to the decision making process. 
- ↑ Müller B, Harbarth S, Stolz D, Bingisser R, Mueller C, Leuppi J, et al. Diagnostic and prognostic accuracy of clinical and laboratory parameters in community-acquired pneumonia. BMC Infect Dis 2007;7:10
- ↑ Müller B, Prat C. Markers of acute inflammation in assessing and managing lower respiratory tract infections: focus on procalcitonin. Clin Microbiol Infect 2006;12(Supplement 9):8-16
- ↑ Briel M, Christ-Crain M, Young J, Schuetz P, Huber P, Periat P, et al. Procalcitonin-guided antibiotic use versus a standard approach for acute respiratory tract infections in primary care: study protocol for a randomised controlled trial and baseline characteristics of participating general practitioners [ISRCTN73182671. BMC Fam Pract 2005;6(34):1-8 ]
- ↑ Huang DT et al. Procalcitonin-Guided Use of Antibiotics for Lower Respiratory Tract Infection NEJM 2018DOI: 10.1056/NEJMoa1802670