Vitamin D (25 Hydroxy)

The concentration of 25-hydroxyvitamin D2 and D3 in serum/plasma are determined by heptane liquid-liquid extraction and liquid chromatography-tandem mass spectrometry. The reportable range is 1 - 200 ng/mL with imprecision between 7.7 and 11.5 %CV.

This is the preferred initial test for assessing vitamin D status in patients without renal disease. See Vitamin D Testing Recommendations for an overview of Vitamin D testing.

There is considerable debate over the range of total 25-hydroxy vitamin D concentrations in serum or plasma that is consistent with optimal health. Currently, there are no consensus guidelines for targeting plasma 25-hydroxy vitamin D concentrations and no studies addressing the outcomes of long-term vitamin D supplementation in general populations. Therefore, we have reviewed the available literature and performed population studies here in Seattle to arrive at our interpretive ranges (see Reference Range section below).

Reference ranges for Total 25-Hydroxy Vitamin D

Studies addressing a recommended lower limit for optimal 25-hydroxy vitamin D concentration have largely relied on surrogate markers such as stabilization of parathyroid hormone concentrations or excretion of calcium in urine. These have determined that 30-32 ng/mL of 25-hydroxy vitamin D is sufficient to prevent calcium wasting and inhibit compensatory mechanisms [reviewed in Holick, 2007]. However, other studies have demonstrated that there is actually a higher rate of hip fracture in populations with 25-hydroxy vitamin D concentrations above 28-32 ng/mL (Looker, 2008 and Cauley, 2008). From the bulk of the available literature it appears that 25-hydroxy vitamin D levels above 20 ng/mL are important for good bone health, but there is insufficient evidence to supplement the diet of all patients with plasma or serum 25-hydroxy vitamin D concentrations between 20 and 30 ng/mL.

Now let us turn to the upper limit of the recommended range. In a population of laboratory personnel in Seattle not taking supplements, the highest concentration of 25-hydroxy vitamin D measured was 43.9 ng/mL (N=40) (Saenger, 2006). From the literature, the highest concentration of 25-hydroxy vitamin D in a person not taking supplements was 64 ng/mL, which was detected in a lifeguard at 30° latitude in the summer months (Vieth, 1999). From this information, it appears that people living in the Pacific Northwest are quite unlikely to have concentrations exceeding 50 ng/mL unless they are taking supplemental vitamin D. In addition to these data, the literature reports the lowest serum concentration of 25-hydroxy vitamin D associated with what appeared to be hypervitaminosis was 88.4 ng/mL (Vieth, 1999). We have decided to be conservative and use >80 ng/mL as our level of possible toxicity. It should be noted that there have been no long-term studies showing the safety and efficacy of vitamin D supplementation to levels above 30 ng/mL in the general population. In fact, the best study so far demonstrated that vitamin D supplementation in patients at higher risk for hip fractures had more kidney stones and no improvement in the rate of hip fracture (Jackson, 2006). Therefore, we include two upper limits in our interpretive comments, one suggestive of supplementation in the Pacific Northwest population (>50 ng/mL) and one that is consistent with possible toxicity (>80 ng/mL).

• Preferred: 4 mL blood in Gold SST tube
• Also Acceptable: 4 mL blood in one of
  • Orange RST
  • Lavender (EDTA)
  • Lime Green PST
  • Red Top
• Pediatric Draw: 1 full Lime Green Microtainer

Note for Pediatric Draw: If ordered at the same time as an Infant Nutrition Panel (INUTP2), 2 microtainers as indicated in the Infant panel are sufficient. No additional microtainers need to be collected.

PST or GOLD SST: centrifuge and refrigerate plasma/serum in original container

LAVENDER or RED top: centrifuge, remove plasma/serum from red cells and refrigerate.

Stability: refrigerated, 7 days.

If samples can’t be analyzed within 7 days: aliquot serum/plasma from spun tube and store at -20C.

NOTE: VITA, VITD and VITE can be run on same aliquot

HMC VITAG and/or VITEG shared sample: place specimen in VITAG/VITEG refrigerated rack.