We read with great interest the recently published article by Franasiak et al in which the authors aimed to characterize the relationship between serum 25-hydroxy vitamin D (25-OH D) levels and implantation and clinical pregnancy rates in women who undergo a euploid blastocyst embryo transfer. They concluded that vitamin D status was unrelated to pregnancy outcomes in women undergoing euploid embryo transfer. Measuring serum 25-OH D levels did not predict the likelihood that euploid blastocysts will implant and these results may not apply to women who do not undergo extended embryo culture, blastocyst biopsy for comprehensive chromosome screening, and euploid embryo transfer. However, we think that there are some points that should be emphasized about this study.

First, as seen in the “Materials and Methods” section of the study by Franasiak et al, patients were selected from the electronic medical record from December 2012 through December 2013. Additionally, authors have used reference ranges, which represent clinical decision values that apply to males and females of all ages. As is known, serum 25-OH D levels are affected by many factors including pigmentation, sunlight exposure, clothing, and the season of the year. The recent study by Kasahara et al revealed that vitamin D levels peak in August and trough in February in the general population of the United States. However, we think that, especially in population-based studies, time of sampling, an important preanalytical factor due to the seasonality of serum vitamin D levels, should be considered for avoiding patient selection bias.

Second, since markedly different methods are used for vitamin D testing across clinical laboratories and reliability of these methods are controversial, the importance of a variety of measurements could be specified in the conclusion. Additionally, if available, authors should have used liquid chromatography-tandem mass spectrometry, which is a state-of-the-art analytical technique for vitamin D measurement and offers advantages of improved specificity and sensitivity.

In conclusion, preanalytical factors such as time of sampling and/or seasonality of the measure and should be considered for the studies including vitamin D measurements and use of state-of-the-art methods for vitamin D measurement would improve credibility of the study.