CDC Reports Decade-Year Effort to Standardize Vitamin D Testing

Vitamin D tests certified by the Centers for Disease Control and Prevention (CDC) Clinical Standards Program (CSP) are generally well calibrated, according to a new study presented at the Endocrine Society’s annual meeting, ENDO 2024, in Boston, Massachusetts. However, the researchers found some inaccuracies among the tests they studied.

The blood tests show “adequate” analytical accuracy, said Oto Sugahara, manager of the CDC’s Vitamin D Standardization and Certification Program (VDSCP) in Atlanta, Georgia, and the study’s lead researcher.

In this case, analytical precision refers to the ability of a test to accurately analyze vitamin D. The laboratory medicine community will decide what level of analytical precision is appropriate, Sugawara said.

She said, “Some inaccuracies appear to be caused by a lack of analytical specificity and other factors. These inaccuracies may lead to misclassification of patients’ vitamin D status.” Analytical specificity is the ability of a test to only measure vitamin D.

Vitamin D is important for bone health, and vitamin D testing is one of the most requested laboratory tests in the United States, Sugawara noted.

“Vitamin D testing can help health care professionals make the right diagnosis and appropriate treatment decisions for their patients,” she said. “However, many clinical organizations and experts have raised concerns about the accuracy and reliability of vitamin D testing.”

According to the CDC, the VDSCP was launched in 2013 to evaluate and improve the accuracy and reliability of vitamin D tests. For vitamin D tests enrolled in the voluntary program, the VDSCP collects performance data and reviews it annually.

“Since 2013, the tests enrolled in the program have overall improved and become better coordinated,” Sugawara said.

She reported that the average calibration bias for all tests in the 2022 VDSCP was less than 1%. Bias is the deviation of test results from the true value as determined by the CDC’s reference method for vitamin D.

Sugahara said U.S. scientists use two main technologies to measure vitamin D in the blood. One is mass spectrometry, which measures 25-hydroxyvitamin D2 and D3 separately and adds up the levels. The other is an immunoassay, which measures both compounds simultaneously and reports the total amount of 25-hydroxyvitamin D as one result.

Sugawara reported that in 2022, the average calibration bias for immunoassays among VDSCP participants was 0.86%, and 0.55% for assays using mass spectrometry.

But, she added, as calibration improved, sample-specific inaccurate results became more apparent.

For example, some tests measure compounds other than 25-hydroxyvitamin D, and Sugahara’s study found that these could falsely inflate the results of some blood samples, leading some tests to erroneously classify results as sufficient for samples with baseline values ​​that would normally indicate vitamin D deficiency, which would require vitamin D supplementation.

“While most vitamin D tests in our program have been improved, inaccuracies remain in some samples, and CDC is working with program participants to address these situations,” Sugahara said.

The VDSCP supports more than 35 tests, laboratories and researchers from approximately 15 countries each year, said co-investigator and CDC Clinical Standardization Program (CSP) director Dr. Hubert Vesper.