25-hydroxy vitamin D testing for funded requests                     

Labtests has been performing all commercial 25-hydroxy vitamin D (Vit D) tests since 2012. As of 24^th June 2019, under a new agreement, we will also be performing all funded Vit D tests in the community.

The method used by Labtests (Abbott Architect) is an immunoassay that has excellent precision and is closely aligned to the gold standard method (LCMS).  Results correlate very well with the Roche immunoassay method used at LabPlus (Roche Cobas), although Labplus results are on average slightly higher by an average of 9.8 nmol/L. 

In the interest of clinical efficiency all funded requests will be vetted, with criteria aligned to those of Labplus.¹ The criteria are also aligned to recommendations from the RCPA².  Vit D will be approved if:

• Ordered by:  endocrinologists, gastroenterologists, bariatric surgeons, rheumatologists, nephrologists, specialist general physicians, orthopaedic surgeons, oncologists, neurologists
• Ordered for specific high risk groups for rickets/osteomalacia (e.g. cystic fibrosis, proven malabsorption, bariatric surgery, refugees, deeply pigmented skin, full veil wearers)
• Ordered for the investigation of rickets/osteomalacia, disorders of calcium and phosphate metabolism, osteoporosis or other metabolic bone disease
• All requests on children (less than 16 yr) will be approved
• Ordered for other patients after discussion with, and approval by, a Labtests Chemical Pathologist
• The clinical indication for testing must be written on the request form.  Requests with no clinical details will be declined.

Patients with isolated ALP elevation should have a P1NP test first, for increased bone turnover.  If P1NP is normal for age, then Vit D is not indicated. 

Patients with low Vit D may be retested not less than 3 months after starting vitamin D supplementation. Once an adequate dosage has been established, no further testing is usually required.

Note that various factors such as levels of vitamin D binding protein (which falls up to 20% during illness), and obesity, can influence plasma results, so they are an imperfect reflection of body stores.  Recent major randomised trials of vitamin D supplementation to reduce risk of cardiovascular disease, cancer and development of diabetes have also not supported benefit for this purpose.^3-5 There was also no benefit of Vit D supplementation in pregnancy and lactation on infant growth.⁶

The expected turnaround time will be 2 days.  However, if the test is rejected the following comment will be released: The requested test has not been performed due to lack of or unsuitability of clinical information. The sample will be stored frozen for 3 weeks. Criteria for funded testing have been aligned to LabPlus. Please email or call the on-call chemical pathologist at Labtests to discuss and perform the test if appropriate.  

The three (3) weeks in which the sample is stored is a window of opportunity to perform the test after discussion with the on-call chemical pathologist if the requestor deems it clinically indicated. However it is advisable to write the clinical indication clearly on the request form right from the start to avoid delays in testing.

Abbreviations:

LCMS: Liquid chromatography mass spectrometry

RCPA: Royal College of Pathologists of Australasia

References:

1. Pathologist Approval for Laboratory Tests Vetting Policy. LabPLUS Test Guide https://testguide.adhb.govt.nz/EGuide
2. Use and Interpretation of vitamin D testing. RCPA Position statement (last approved May 2019). https://www.rcpa.edu.au/Library/College-Policies/Position-Statements/Use-and-Interpretation-of-Vitamin-D-Testing (last accessed 19/6/19)
3. ³Man JE, Cook NR, Lee I-M, et al. Vitamin D supplements and prevention of cancer and cardiovascular disease.  N. Engl. J. Med. 2019; 380:33-44.  (Jan 3 2019) DOI: 10.1056/NEJMoa1809944
4. ⁴Goulao B., Stewart F, Ford JA, MacLennan G, Avenell A. Cancer and vitamin D supplementation: a systematic review and meta-analysis. Am J Clin Nutr 2018; 107:652-63
5. ⁵Pittas AG, Dawson-Hughes B, Shehan P, et al. Vitamind D supplementation and preventino of type 2 diabetes.  N. Engl. J. Med. (June 7 2019).  DOI: 10.1056/NEJMoa1900906
6. Roth DE, Morris SK, Zlotkin S, et al. Vitamin D supplementation in pregnancy and lactation and infant growth.  N. Engl. J. Med. 2018; 379: 535-46. DOI: 10.1056/NEJMoa1800927