Vitamin D Testing: Uncovering Potential Inaccuracies

Despite general improvements since its inception in 2013, the Vitamin D Standardization-Certification Program (VDSCP) has continued to highlight inconsistencies in the accuracy of vitamin D testing, leading to persistent patient misclassification beyond 2022. Knowledge of these inaccuracies are crucial for providers to further improve vitamin D assays and avoid misclassification.1

“Vitamin D tests help health care providers make a correct diagnosis and appropriate treatment decisions for patients. However, many clinical organizations and experts have expressed concern about the accuracy and reliability of vitamin D assays,” said Otoe Sugahara, manager of the CDC’s VDSCP, in a press release.2

The main function of the VDSCP is to review and assess the accuracy of vitamin D testing, collecting assay data once a year.

“Since 2013, the assays enrolled in the program have generally improved and become better calibrated,” continued Sugahara.2

Key Takeaways The Vitamin D Standardization-Certification Program (VDSCP) has continued to highlight inconsistencies in the accuracy of vitamin D testing.

Researchers aimed to address the CDC's growing improvements in vitamin D testing over the years while also identifying the potential for misclassifications.

However, as vitamin D test reviews continue year after year, misclassifications persist as researchers are unable to follow Endocrine Society guidelines. According to the Endocrine Society, vitamin D levels of 50 nmol/L or less are considered a deficiency, while levels of 50 nmol/L to 75 nmol/L are considered an insufficiency.1

Researchers used these guidelines in a sample test with a vitamin D level of 55.1 nmol/L, which would be classified as insufficient, but 44% of results reported a deficiency. With misclassifications and inaccurate results, researchers have used calibration biases, whether positive or negative, to assess the accuracy of vitamin d assays.

Despite general improvements since its inception in 2013, the Vitamin D Standardization-Certification Program (VDSCP) has continued to highlight inconsistencies in the accuracy of vitamin D testing. | image credit: ratmaner / stock.adobe.com

“The program has seen improvements in calibration bias since its inception. In 2022, mass spectrometry-based assays had a mean bias of 0.55% and immunoassays had a mean bias of 0.86%, for all assays enrolled in VDSCP. This indicates that overall, the assays enrolled in the VDSCP are well-calibrated. However, with improvements in calibration, sample-specific inaccurate results became more apparent,” wrote the authors of an abstract presented at ENDO 2024, the annual meeting of the Endocrine Society.1

READ MORE: Vitamin D and COVID-19: An Evidence-Based Review

Indeed, findings part of the VDSCP’s most recent review show that calibration has improved but is still sporadic and not always 100% accurate. So, for researchers involved with the VDSCP, it’s important to get as close to this accuracy threshold as possible.

Furthermore, these inconsistencies have existed in vitamin D testing since the beginning of the VDSCP. A previous study analyzing the accuracy of liquid chromatography-tandem mass spectrometry assays reported on misclassifications and inaccuracies almost a decade ago.

“Reliability (precision) and accuracy are of particular concern in epidemiological studies where poor precision leads to non-differential misclassification bias and may cause null findings. Furthermore, research on the risks of vitamin D deficiency for human health outcomes often involves combining the results of many different studies, where accuracy may be a particular issue,” wrote authors of the study.3

These findings highlight a need for better accuracy within the VDSCP. With the improvement of testing biases, the CDC was able to more easily identify sample-specific inaccuracies; and it’s these specific inaccuracies that researchers are addressing in the abstract.

From improvements over the years to new-found inconsistencies that have only recently came to light in the VDSCP, it is now the CDC’s goal to maximize overall assay accuracy while avoiding misclassifications as much as possible.

“While most vitamin D tests are effective, it is important for healthcare providers to be aware of the potential inconsistencies associated with vitamin D tests to avoid misclassification of the patients,” concluded authors of the abstract.1

READ MORE: Vitamin D Deficiency May Increase Gastrointestinal Bleeding Risk in Patients on Blood Thinners

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References

1. Sugahara O, Buchannan T, Danilenko U, et al. CDC’s efforts in standardizing of vitamin D tests: challenges and potential patient misclassifications. Presented at: ENDO 2024; June 1-4; Boston, MA.

2. CDC reports its decade-long efforts in standardizing vitamin D tests. News release. The Endocrine Society. June 2, 2024. Accessed June 6, 2024.

3. Black LJ, Anderson D, Clarke MW, Ponsonby AL, Lucas RM; Ausimmune Investigator Group. Analytical Bias in the Measurement of Serum 25-Hydroxyvitamin D Concentrations Impairs Assessment of Vitamin D Status in Clinical and Research Settings. PLoS One. 2015;10(8):e0135478. Published 2015 Aug 12. doi:10.1371/journal.pone.0135478