Vitamin D testing in primary care has surged over the past decade. In England alone, the number of tests ordered through general practice rose sharply between 2008 and 2018, driven by growing awareness of deficiency and its links to conditions ranging from rickets and osteomalacia to respiratory infections, cardiovascular disease, and some cancers.
Point-of-care tests – portable devices that can return a result within minutes – could reshape how that demand is met. For the groups at highest risk of vitamin D deficiency, including older adults, people with darker skin pigmentation, children, and those with chronic conditions such as diabetes or kidney disease, the appeal is clear: faster diagnosis, fewer hospital visits, and the possibility of testing and treatment in a single consultation.
But the clinical evidence that these devices actually work at the point of care barely exists.
An evidence base of 78 samples
A systematic review by researchers at the University of Oxford and Birmingham Women's and Children's NHS Foundation Trust, published in Diagnostics and funded by the NIHR HealthTech Research Centre in Community Healthcare, screened nearly 1,500 papers for evidence about vitamin D point-of-care testing. Only four studies met the criteria for clinical evaluation at the point-of-care – instances where the device was actually used and processed next to a patient, rather than on samples sent to a laboratory.
Those four studies, carried out in the UK, Malta, Portugal, and the USA, assessed five different devices against laboratory reference standards. The total number of samples across all comparisons was 78, with individual studies ranging from just 6 to 20 samples each.
The reason so few studies qualified is telling. In most published research, the "point-of-care" device had actually been used on samples processed in a laboratory – not at a patient's side. A test validated under laboratory conditions may not perform the same way in a GP surgery, a clinic, or a patient's home, precisely the settings where these devices are intended to be used.
"There is a well-recognised gap between developing a point-of-care test and demonstrating that it works reliably in practice," said Dr Thomas Fanshawe, Senior Statistician at the University of Oxford and one of the review's authors. "What surprised us was how wide that gap is for vitamin D. Several devices are available or in development, but the clinical evidence for their accuracy when used at the point of care is almost entirely absent."
Inconsistent devices, incomplete picture
The few devices that have been tested in clinical settings show troubling variation. One closely matched laboratory results. But another inflated patients' vitamin D levels by up to a third – a discrepancy large enough to falsely reassure someone who is actually deficient. In a third evaluation, five out of seven pregnant women would have received the wrong classification of Vitamin D deficiency or insufficiency based on the point-of-care result. All of these assessments were limited by small sample sizes.
Much of even this thin evidence is difficult to rely on. Key details were often missing from the published results – the characteristics of the patients, how they were recruited, whether the point-of-care and laboratory tests were performed close enough together in time to be comparable.
What good evaluation looks like
The findings amount to a practical checklist for researchers and industry partners developing vitamin D point-of-care tests:
- Evaluate in clinical settings. Process the test at the point of care, not in a laboratory, so that accuracy estimates reflect real-world performance.
- Recruit adequate sample sizes. Studies of 6 to 20 samples cannot establish whether a test is fit for purpose.
- Report study design fully. Participant characteristics, recruitment methods, reference standard details, and the timing between tests all need to be transparent.
- Include relevant populations. The clinical case for point-of-care vitamin D testing rests on reaching people at higher risk of deficiency. Future studies should be designed to reflect this.
Dr Chris Bird, an emergency medicine clinician at Birmingham Women's and Children's NHS Foundation Trust and a co-author of the review, said the findings had direct implications for clinical decision-making. "Clinicians are seeing more patients with suspected vitamin D deficiency, and a reliable point-of-care test would make a real difference – particularly for children, where avoiding an unnecessary venous blood draw matters," he said. "But without robust evidence from clinical settings, we can't be confident that these devices are giving us accurate results. That's a risk for patients."
The full paper is available in Diagnostics https://doi.org/10.3390/diagnostics16081129