In the UK, requests for vitamin D blood tests have risen over the past few years1, to the extent that testing for vitamin D deficiency has become commonplace in clinical practice, despite there being uncertainty surrounding who should be tested, what the results mean in context, and whether a person’s low vitamin D levels are themselves a causal or surrogate marker.1 This guide summarises the most recent advice from the National Institute for Health and Care Excellence (NICE), the Royal Osteoporosis Society (ROS), the Scientific Advisory Commission on Nutrition (SACN) and Public Health England on the subject of vitamin D testing.
Who should be tested?
Put simply, most people in the general population do not require routine testing for vitamin D deficiency.2 The recommendation on who to test is based on a combination of expert opinion in the ROS guideline ‘Vitamin D and bone health: a practical clinical guideline for patient management’ and the NICE guideline ‘Vitamin D: increasing supplement use in at-risk groups’. The official guidance says that practitioners should only test vitamin D status if someone has symptoms of deficiency or is at very high risk of being deficient.3
In adults, clinical reasons or indications for the test can include:4
Suspected osteomalacia, whose symptoms may include muscle weakness and aching, pain in the bones, myopathic (or waddling) gait, impaired physical function and pain in the lower back
If the patient has had a fall
Suspected hypocalcaemia, whose symptoms can include numbness and cramping in the muscles
The patient presenting with chronic widespread pain
Diagnosis of a bone disease, for example osteoporosis or Paget’s disease, whose outlook may be improved by treatment with vitamin D
Cases in which correcting vitamin D deficiency would be appropriate prior to other specific treatments
In children under 18 years old, clinical reasons or indications for a vitamin D test include:
Suspected rickets, whose symptoms can include bow legs, knock knees, painful swelling of the wrists, craniotabes or teeth being slow to erupt
The patient being diagnosed with a chronic disease, particularly those causing malabsorption, that may put them at heightened risk of vitamin D deficiency
The patient presenting with muscle weakness or bone pain that cannot otherwise be explained
The patient being diagnosed with a disease, for example osteogenesis imperfecta, where bringing vitamin D up to optimal levels prior to specific treatment would usually be indicated
How is vitamin D deficiency tested?
Experts are in agreement that a venous blood test to measure serum 25-hydroxyvitamin D (25(OH)D) is the best way of gauging vitamin D levels in patients.6 The reasons for this are that this blood test reflects both dermal and dietary sources of vitamin D, has a relatively long half life of around two to three weeks, and is not subject to tight homeostatic control, with the result being that the test gives a clear indication of the patient’s vitamin D availability over the weeks preceding being tested for deficiency.7 The results are measured by checking the levels of 25(OH)D in nanomoles per litre (nmol/l).
Vitamin D test results
For healthcare professionals, it is important to understand the results of the 25(OH)D test in context, as they can vary based on factors such as the patient’s age, weight or sex, and even the varying testing thresholds in place in different laboratories.7 In this regard, NICE advises that there is no current consensus on when to diagnose vitamin D deficiency, although NICE2, the SACN7 and the Institute of Medicine8 agree the following reasonable thresholds for levels of 25(OH)D when defining a patient’s vitamin D status.
Repeat vitamin D blood tests after treatment
Follow-up tests and their frequency vary based on which clinical guidelines are being followed. NICE advises checking vitamin D levels again within three to six months of a loading dose being prescribed.2 It is important that this is not done sooner than the three-month mark as it takes at least this long for vitamin D levels to stabilise. The ROS, meanwhile, recommends that follow-up testing is unnecessary unless the patient is suffering from malabsorption or remains otherwise symptomatic.4
Depending on the patient, individual practitioners should decide which of the NICE or ROS guidelines to follow. In any case, the general advice is that long-term routine monitoring is not required. However, adjusted serum calcium should be checked one month after completing the loading regimen or after starting vitamin D treatment in case primary hyperparathyroidism has been unmasked.4
Job Code: FUL-519 Date of Preparation: June 2019 Date of Revision: January 2020