Title:Vitamin D Measurement in the Intensive Care Unit: Methodology, Clinical Relevance and Interpretation of a Random Value
VOLUME: 12 ISSUE: 4
Author(s):Anand Krishnan and Bala Venkatesh
Affiliation:Princess Alexandra Hospital, Ipswich Road, Woolloongabba, QLD 4102, Australia.
Keywords:Cardiovascular, cytokines, immunity, infection, inflammation, muscle, sepsis.
Abstract:Vitamin D deficiency, as measured by a random level of 25-hydroxyvitamin D is very prevalent in critically ill
patients admitted to the ICU and is associated with adverse outcomes. Both 25(OH)vitamin D and 1α,25(OH)2D3 are
difficult to analyse because of their lipophilic nature, affinity for VDBP and small concentrations. Also, the various tests
used to estimate vitamin D levels show significant inter- and intra-assay variability, which significantly affect the veracity
of the results obtained and confound their interpretation. The two main types of assays include those that directly estimate
vitamin D levels (HPLC, LC-MS/MS) and competitive binding assays (RIA, EIA). The former methods require skilled
operators, with prolonged assay times and increased cost, whereas the latter are cheaper and easy to perform, but with
decreased accuracy. The direct assays are not affected by lipophilic substances in plasma and heterophile antibodies, but
may overestimate vitamin D levels by measuring the 3-epimers. These problems can be eliminated by adequate
standardization of the test using SRMs provided by NIST, as well as participating in proficiency schemes like DEQAS. It
is therefore important to consider the test employed as well as laboratory quality control, while interpreting vitamin D
results. A single random measurement may not be reflective of the vitamin D status in ICU patients because of changes
with fluid administration, and intra-day variation in 25-hydroxyvitamin D levels. 1α,25(OH)2D3 may behave differently to
25-hydroxyvitamin D, both in plasma and at tissue level, in inflammatory states. Measurement of tissue 1α,25(OH)2D3
levels may provide the true estimate of vitamin D activity.
