Journal of Medical Case Reports volume 13, February 2019
Christine B. French, Sharon L. McDonnell & Reinhold Vieth
- Vitamin D measurements vary with the same sample of blood – March 2014
- 1090 tests of the same blood sample (figure 1)
- Percent who are Vitamin D Deficient: 6, 9, or 22 – depends on testing system – Oct 2014
- Technical comparison of vitamin D testers – July 2013
- Vitamin D tester falsely said half of hemodialysis patients had less than 30 ng – Jan 2013
- Low cost vitamin D Blood Tests
- Problems with Vitamin D Testing – chapter – Aug 2019
- Big differences (~10 ng) in vitamin D test results, even when using the same type of tester – Aug 2015
- Huge differences in % deficient (< 30ng) depending on the tester used
- Note: A, B, and C unable to detect/measure < 13 nmol (5 ng)
Download the PDF from VitaminDWiki
3 days for a person who had been taking 5,000 IU of Vitamin D daily at noon for a year
Note: Portion showing a sick day was removed from this chart
Vitamin D nutrition research requires accurate measures of circulating 25-hydroxyvitamin D. Our objectives were to test whether a diurnal fluctuation in blood-spot concentrations of 25-hydroxyvitamin D can be demonstrated statistically in a single individual, and whether such fluctuation is affected by the pre-dose versus post-dose timing of the blood draw.
The participant in this case study was a generally healthy Caucasian woman in her 40s who has taken 5000 IU vitamin D3 supplement at midday for over 1 year. Each blood sample was drawn individually from a finger prick onto filter paper at morning, midday, or night, on 4 days (three groups of five individual blood samples per collection day). On days 1 and 2, the midday samples were collected approximately 1 hour after the supplement was taken; on days 3 and 4, the midday samples were collected within an hour prior to supplementation (the classical, daily “trough” value for a drug). There was a significant daily pattern of variation in 25-hydroxyvitamin D concentrations (analysis of variance p ≤ 0.02 for 3 of the 4 days): peak midday mean 25-hydroxyvitamin D was approximately 20% higher than in the morning, and approximately 13% higher than in the evening. Trough sampling produced no significant difference in 25-hydroxyvitamin D compared to sampling an hour after the dose. An incidental finding was that acute illness during the study was related to acutely lower 25-hydroxyvitamin D at every sampling time in the day (p < 0.00001).
There was a consistent diurnal variation in 25-hydroxyvitamin D, with the peak at midday. There was no difference between trough versus post-dose blood draws. Acute illness may acutely lower serum 25-hydroxyvitamin D levels. Because within-person, within-day variability in 25-hydroxyvitamin D is approximately 20%, sampling time introduces systematic error in vitamin D nutritional assessment that is bigger than random analytical error or choice of assay method.
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