Many thanks for all the replies to my query two months ago regarding the 30
year old lady whose total calcium was 1.8 mmol/L and whose ionised calcium
was discrepant with this at 1.52 mmol/L. She was presumed to have idiopathic
hypoparathyroidism and was on Vit D treatment.
Special thanks go to Dermot Neely who suggested that this girl may have
hypocalcaemia with hypercalciuria due to a mutation in her calcium sensing
receptor (NEJM 1996; 335: 1115-22. Certainly, her biochemical presentation
fits this condition exactly (although there is no mention of any
total/ionised discrepancy). Genetic analysis is on its way to London.
Interestingly, the authors of the NEJM paper could not explain the
underlying mechanism responsible for the hypercalciuria and nephrocalcinosis
that occurs in these patients with vitamin D therapy when their total
calcium would suggest they are hypocalcaemic. Perhaps this discrepancy
between total/ionised calcium is responsible, especially since we now know
her mother, who is not on treatment, also has a total calcium of 1.8 mmol/L
but a more consistent ionised calcium of 0.9 mmol/L.
Needless to say this requires further investigation.
Dr. Eric S. Kilpatrick
Consultant in Chemical Pathology
Department of Clinical Biochemistry
Hull Royal Infirmary
Anlaby Road
Hull HU3 2JZ
Tel 01482-674312
Fax 01482-674310
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