Endocrine Abstracts

Aim: Population studies have shown that basal calcitonin concentrations are below 10 ng/l in the normal population. Ten nanograms per liter is used as a diagnostic cut-off by most laboratories and patients with levels higher than this are offered a pentagastrin stimulation test to exclude medullary thyroid carcinoma. However, these guidelines were adopted from studies that used the Cisbio Immunoradiometric assay for measuring calcitonin. Most laboratories have now moved to chemiluminescent assays. Our laboratory uses the Diasorin Liaison chemiluminescent. We assessed the validity of continuing to use 10 ng/l as a diagnostic cut-off for our assay.

Methods: Sixty-one healthy, volunteers (35 female and 26 male) were recruited (07/Q0406/18). Age ranges were 21–64 years (mean=39.9) for males and 19–58 years (mean=30.7) for females. A single basal blood sample was collected into plain tubes and allowed to clot. Samples were immediately centrifuged, stored at −20  °C and assayed using the Diasorin Liason Calcitonin assay.

Results: The manufacturers quote a reference range of <5.5 ng/l for females and <18.9 ng/l for males. Our study showed that the upper limit for males (22 ng/l) is similar to the manufacturer’s range. However, the value for females (10.8 ng/l) is double the recommendation but is in keeping with observations made in our clinical practice and similar to the reference ranges quoted by manufacturers of most other calcitonin assays. Two out of 35 females and 7 out of 26 males had levels above 10 ng/l.

Conclusion: Our study revealed that 6% of females and 27% of normal males had levels greater than 10 ng/l. This suggests that adhering to a threshold of 10 ng/l for pentagastrin stimulation test will result in a significant number of patients with normal results being subjected to a pentagastrin test unnecessarily. We therefore recommend that the old diagnostic cut-off of 10 ng/l be reviewed.