The (Sun)Light and Dark of 25-Hydroxyvitamin D Testing

Serum 25-Hydroxyvitamin D Concentrations from Patient Groups at High Risk for Deficiency

BACKGROUND

Deficiency of 25-hydroxyvitamin D in serum is endemic in the general population, and testing for this hormone is useful in accessing a patient's overall health and well-being.

METHODS

We obtained blood from 216 hospitalized patients and outpatients divided into 4 groups thought to be at high risk of 25-hydroxyvitamin D deficiency: homeless, recreational drug abusers, psychiatric patients with limited access to the outdoors, and those infected with HIV. The 25-hydroxyvitamin D concentrations from these patients were determined with 2 different methodologies (immunoassay and mass spectrometry) and compared against 25-hydroxyvitamin D concentrations in apparently healthy controls. We hypothesized that these groups may be at higher risk for vitamin D deficiency because of poor nutrition, inadequate housing, restricted access to outdoors, or the presence of chronic disease.

RESULTS

For each of the patient groups including healthy controls, the median concentration of 25-hydroxyvitamin D was below 30 ng/mL, indicating deficiency. Comparisons between the healthy controls and the other groups were not statistically significant with either methodology, except for the homeless patients in whom a higher number of individuals had 25-hydroxyvitamin D concentrations below 20 ng/mL. Results between the 2 testing platforms demonstrated that only 52% of the specimens analyzed by immunoassay agreed within ±10% of the LC-MS/MS results, with an overall correlation coefficient to 0.920. The degree of concordance for deficiency with 2 published cutoffs of 20 and 30 ng/mL was 91% and 91%, respectively.

CONCLUSIONS

Vitamin D deficiency is a common finding in all the populations studied. The Lumipulse^® G vitamin D immunoassay is an alternative for detecting vitamin D deficiency.

Evaluation of a New Generation Automated Assay for 25-Hydroxy Vitamin D Based on Competitive Protein Binding

BACKGROUND

The interest for vitamin D has exponentially increased testing demand for 25-hydroxy vitamin D [25(OH)D]. Consequently, many laboratories are switching from LC-MS/MS methods to automated, high-throughput immunoassays. One of the major potential issues with these assays has been the lack of cross-reactivity with 25(OH)D2.

METHODS

We have evaluated the Roche Elecsys vitamin D total II assay for accuracy by comparing 79 patient samples with LC-MS/MS. The cross-reactivity for 25(OH)D2 was evaluated by analyzing samples with high 25(OH)D2 separately and estimating 25(OH)D2 recovery, as well as by spiking of 25(OH)D2. The assay was further evaluated for precision, linearity, sample type, and common interferences.

RESULTS

There was mostly good agreement between the Elecsys and LC-MS/MS assays (Deming regression: y = 0.95x + 0.70), with an overall bias of 2.3% (-0.84 ng/mL). However, there were 6 out of 79 (7.6%) discordant samples. The Deming regression for samples with high 25(OH)D2 compared to LC-MS/MS showed similar slope and intercept (y = 0.97x - 1.1). The average recovery of 25(OH)D2 for these samples was 90%. The initial precision studies were in general agreement with the package insert, but long-term clinical use showed higher-than-claimed imprecision (11.7%-14.4% at 12 ng/mL and 6.9%-7.6% at 27 ng/mL; claimed: 7.2% and 5.0%, respectively). We observed 1 falsely high result in plasma, an issue previously addressed by Roche in a medical device correction.

CONCLUSIONS

The analytical performance of the Roche Vitamin D assay was acceptable, and the assay had a good cross-reactivity for 25(OH)D2.