Vitamin D tests certified by the U.S. Centers for Disease Control and Prevention Clinical Standardization Programs have generally improved and become better calibrated since 2013, according to research presented at ENDO 2024, the Endocrine Society's annual meeting in Boston, Mass.

"Since 2013, the assays enrolled in the program have generally improved and become better calibrated," said Otoe Sugahara, manager of the U.S. Centers for Disease Control and Prevention (CDC) Vitamin D Standardization-Certification Program (VDSCP) in Atlanta, Ga. "However, some inaccuracies appear to be caused by a lack of analytical specificity and other factors. These inaccuracies may result in the incorrect classification of patients' vitamin D status."

The VDSCP began in 2013 to help assess and improve the accuracy and reliability of vitamin D tests, which are among the most requested laboratory tests in the U.S. For vitamin D assays enrolled in the voluntary program, the VDSCP collects and annually reviews their performance data.

In 2022, the average calibration bias of VDSCP participants for immunoassays was 0.86%, and for assays using mass spectrometry, it was 0.55%, Sugahara reported. Bias is the deviation of the test result from the true value, which is determined with the CDC's reference method for vitamin D.

However, with improved calibration, sample-specific inaccurate results became more apparent. Some assays measure other compounds besides 25-hydroxyvitamin D, which Sugahara's research found can falsely elevate the results of some blood samples. Thus, for a sample with a reference value indicating vitamin D deficiency, which typically requires vitamin D supplementation, some tests misclassified the result as sufficient.

"Though most vitamin D tests in our program have improved, there still remain some sample-specific inaccuracies. The CDC is working with program participants to address these situations," Sugahara said. The VDSCP supports more than 35 assays, laboratories, and researchers from approximately 15 countries annually.

The researchers acknowledged some limitations in their study. The laboratory medicine community decides which level of analytical accuracy is appropriate, and the study focused on assays enrolled in the voluntary VDSCP, which may not represent all vitamin D tests available.

Further research is needed to address the sample-specific inaccuracies and ensure reliable vitamin D testing for accurate diagnosis and treatment of patients.

No conflicts of interest were disclosed.