Investigation of seasonal reference intervals for 25-Cholecalciferol (Vitamin D)
A Donaldson, YH Leung, DA Jackson, PA Hill & RS Chapman
Vitamin D is essential to the control of calcium homeostasis and is mainly provided by exposure to sunlight. In the UK circulating Vitamin D is subject to seasonal variation with peak values occurring in early Autumn and trough values in late Spring. Requests for Vitamin D assays are increasing but few laboratories quote seasonal reference ranges and many commercial suppliers quote reference ranges for populations living in warmer, sunnier climates. This study established seasonal reference intervals for Spring and Autumn in our local West London population.
During early October 2002 and early April 2003 approximately 200 sera were selected from samples on the basis of normocalcaemia, normal parathyroid hormone values and an absence of renal disease. Samples were stored at minus 20 degrees centigrade until assay. Vitamin D was assayed by the Nichols Advantage method (automated, direct, chemiluminesence immunoassay). Reference intervals were calculated using medians, 5th and 95th centiles.
Results were 5-112 nanomoles per litre and 7-125 nanomoles per litre for April and October respectively.
Whilst these results demonstrate seasonal variation they are inconsistent with the reference intervals currently in use in our laboratory (15-100 nanmoles per litre). Local clinical opinion suggests that patients with Vitamin D values below 20 nanomoles per litre are deficient and require supplementation. These analytical data suggest that either the cut off of 20 nanomole per litre is contentious or that a proportion of the local West London population may be Vitamin D deficient.