We tried five high street vitamin D self-test kits and got very different results

A Which? investigation into rapid DIY vitamin D tests has raised concerns about accuracy and suitability for consumers after the tests returned a wide variety of results, despite being done on the same day using samples from the same person.

The cut-off level defining sufficient vitamin D levels for these tests also differs from typical NHS and private blood test cut-offs, causing potential confusion for users.

We bought five big-brand, widely available DIY Vitamin D rapid finger-prick self-test kits from Boots, Berkeley Health, MyHealthChecked, OneStep and Newfoundland and compared the results with a private venous blood test taken on the same day at a Care Quality Commission (CQC) registered provider.

We conducted each rapid test on the same person, following the instructions and using a different finger for each kit.

The private blood test showed vitamin D levels well within the sufficient range, but the DIY tests varied widely. We got three different results from the five self-test kits, ranging from very deficient to sufficient vitamin D levels.

Different cut-offs to NHS tests

The NHS and most private labs deem anything higher than a reading of 50nmol/L as a sufficient level of vitamin D, but the rapid tests use a higher cut-off of 75nmol/L.

By this measure, although our tester was well within the sufficient range as defined by the NHS, we would expect them to get an 'insufficient vitamin D' result on the self-test kits.

However, in our snapshot investigation, only the tests from MyHealthChecked and OneStep suggested 'insufficient' levels. The Newfoundland and Berkeley Health test kits indicated our tester had 'deficient' levels of vitamin D, which is markedly lower than the level indicated by the private lab blood test. A deficient result indicates vitamin D levels between 0-25nmol/l, according to the instruction leaflets.

The Boots own-brand self-test, meanwhile, indicated our tester had a sufficient level of vitamin D. Same person, very different results.

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Boots owns MyHealthChecked, and these two vitamin D tests appeared to be the same, based on their product reference numbers, manufacturer details and supporting performance data. However, they produced conflicting results, with one suggesting our tester had good vitamin D levels, and the other indicating they were lacking.

Self-testing kits have become widely available in recent years, with pharmacies, supermarkets and online shops selling them. Their increased popularity has been fuelled by issues with NHS waiting times for appointments, and increased consumer interest in health and wellbeing.

But our findings raise questions about how useful they really are for consumers, and whether they are being regulated appropriately.

Tests tricky to use with ‘inherent variability’ 

Rapid vitamin D tests work by analysing a sample of blood from a finger, which is sucked into a pipette and dispensed into a test cassette that looks like a Covid test. Two lines then appear: a control line and a test line. The test line is read against a small colour card that's included within the pack. For vitamin D, the darker the line, the lower the level of vitamin D detected. It gives one of four results: excess, sufficient, insufficient or deficient.

The way blood is collected can affect the result. For example, if the finger is squeezed too tightly, it might affect the quality of the sample going into the pipette and potentially influence the result. Or if you use too much or too little blood, that also risks altering the result.  

We took great care to conduct the tests exactly as instructed, using a different finger for each test to reduce the chance of taking repeat samples from the same place, which could affect results. 

Even though we suspected there may be issues based on what we already knew about self-test kits, we were surprised by the amount of variation in the results. We also found it tricky to get the hang of the tests and interpret the results correctly.

Birmingham University professors Dr Clare Davenport and Professor Jon Deeks have raised concerns about the suitability of these types of self-tests for consumer use before, warning they can be tricky to use and calling for tighter regulation.

In 2025 they published an analysis of a wide range of commercially available self-testing kits, including tests for vitamin deficiencies, and reported problems with equipment, issues with the sampling process of some tests, and with instructions for use and interpretation of results.  

Dr Davenport said our research highlighted the ‘inherent variability’ of self-testing kits because they rely on an untrained individual taking a finger-prick sample, rather than a professional taking a venous blood sample in a controlled environment. 

‘This is a vitamin D test so the potential detriment may not be as severe, but if this was for cancer where the consequences of a variation of a result would be greater that would be quite scary,’ she added.

‘Well-evaluated self-tests like pregnancy tests and HIV tests have been around for a long time, and we wouldn’t want to dissuade people from using rapid tests in general. But there is clearly a lack of guidance around which type of tests are trustworthy and which are not, and that’s a problem.’

‘When we looked into vitamin D tests,’ Professor Deeks added, ‘I remember thinking – I really can’t tell the difference between this pink and that pink. It’s not like comparing red against green.

‘And if the process of choosing fingers [to test with] has somehow produced some variability in the results found by Which? then capillary blood sampling is not reliable,’ he added.

Whereas some tests seek to give a consumer a simple positive or negative result, such as with a Covid test, part of the challenge with vitamin D testing, they both said, is that the exact concentration in the blood is important. A venous blood draw can provide a precise measurement, but a rapid lateral flow test places results into broad categories such as deficient, insufficient or sufficient. It cannot offer the same precision as a lab test. 

The frustration with test usability has been mirrored in some online reviews for all of the tests we looked at. 

For example, when we checked the Boots website, its rapid vitamin D test had a review star rating of 2.9 out of 5. Several reviewers complained the tests were hard to read, and three said they got a deficient result from a rapid test, but then a GP-ordered NHS blood test confirmed their vitamin D was within the normal range.

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For screening, not diagnosis

These kits serve one purpose for customers: to give you an indication of whether you have an issue, such as a nutrient deficiency. But despite claiming high accuracy rates, companies told us they are a screening tool, not a diagnostic test, and should always be followed up to confirm the result. Which begs the question: what's the point of paying for one?

Our analysis of the vitamin D test packs shows the Boots, MyHealthChecked, OneStep and Newfoundland tests were all manufactured by the same company. Each stated the same accuracy percentage of 94.4%, based on an in-house clinical study of 90 blood samples. Just four of those samples, according to the patient information leaflets, were vitamin D deficient.  

This in-house clinical trial may form part of wider performance evidence. However, the manufacturers did not respond when we asked for comment.

We asked the brands to clarify whether they considered a sample of four people to be sufficient. None of the companies shared specific information on performance evidence with us.

Boots, which also owns the MyHealthChecked brand, said both tests were ‘compliant with the relevant regulations’ and that self-test kits were intended as a screening option rather than a laboratory test.

Newfoundland said: ‘We recognise the point regarding the number of deficient participants, but the study forms part of a wider body of performance evidence considered within the regulatory assessment of the device.’

The company said because a venous blood test and a rapid test were not the same, the results were not directly equivalent.

‘There are a number of factors that may contribute to differences between results, including sample collection, sample volume, test procedure, timing and other procedural factors. Without having observed the testing conditions and procedure used in this particular case, it would not be possible to attribute the difference to any specific factor.’

Berkeley Health said the clinical evidence base was ‘entirely sufficient because it fulfils the rigorous compliance benchmarks required to achieve full CE marking for self-testing rapid screening tests.’

The company said it understood any ‘discrepancy’ could be ‘frustrating for a consumer’ but that its tests were designed as a screening tool, not a diagnostic test and that it advises consumers to have a formal follow-up test.

‘The natural biological variations in a finger prick, combined with the standard testing margins of a home kit, can easily cause the result to lean into the adjacent category,' it said. 

OneStep did not respond.

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Experts ‘not surprised’ by variation in results  

We shared our findings with the president of the Royal College of Pathologists, Dr Bernie Croal. The Royal College of Pathologists represents clinical specialists who analyse blood.

He said he was not surprised by the results, noting that by its nature a finger-prick test conducted at home could not mirror the ‘very precise procedures and quality control oversight’ that take place after a GP venous blood test analysed in a laboratory. 'What this will do is it will create a population of patients who will then go to their GP, who will then feel obliged to repeat the testing.'

On the other hand, he said, people might be tempted to buy expensive supplements needlessly.

‘Vitamin D testing can lead to harm. If people go on vitamin D tablets and the dose is too high, that can lead to high calcium levels in the blood with an increased risk of kidney stones,’ he said.

Self-testing devices are regulated by the Medicines and Healthcare Regulatory Authority (MHRA). We shared our findings and the MHRA said it would assess the evidence. It also said the MHRA was reviewing existing regulations, with proposed changes set to ‘elevate the standard of self-tests'.

Rob Reid, MHRA Deputy Director of Innovative Devices, said that vitamin D tests could ‘help individuals take a more active role in managing their health but that ‘consumers should seek advice from an appropriately qualified healthcare professional to ensure that test results are interpreted in the correct clinical context.’

He urged anyone who had experienced a potentially faulty product or who had concerns about specific devices to notify the MHRA directly at 'devices.compliance@mhra.gov.uk' or via its Yellow Card scheme. 

Sue Davies, Which? head of consumer rights and food policy, said: 'It is deeply concerning that tests sold by major high street brands and online gave completely contradictory results. 

'An inaccurate result risks driving people to spend money on potentially harmful high-dose supplements, or leaving a genuine deficiency untreated.

'The MHRA must raise the bar and tighten regulations so unreliable tests cannot be sold, whether you are buying them in-store or online. Retailers, marketplaces and manufacturers must also step up: any claim about a test’s accuracy must be backed by robust evidence, with instructions that make using the test and interpreting the results completely foolproof.'

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